diff --git a/src/bsp.cpp b/src/bsp.cpp
index bba7265..485902b 100644
--- a/src/bsp.cpp
+++ b/src/bsp.cpp
@@ -39,7 +39,7 @@ SBsp3 *SBsp3::FromMesh(SMesh *m) {
 Vector SBsp3::IntersectionWith(Vector a, Vector b) {
     double da = a.Dot(n) - d;
     double db = b.Dot(n) - d;
-    if(da*db > 0) oops();
+    ssassert(da*db < 0, "Expected segment to intersect BSP node");
 
     double dab = (db - da);
     return (a.ScaledBy(db/dab)).Plus(b.ScaledBy(-da/dab));
@@ -104,7 +104,7 @@ void SBsp3::InsertHow(int how, STriangle *tr, SMesh *instead) {
             more = m;
             break;
         }
-        default: oops();
+        default: ssassert(false, "Unexpected BSP insert type");
     }
     return;
 
@@ -144,7 +144,7 @@ void SBsp3::InsertConvexHow(int how, STriMeta meta, Vector *vertex, int n,
             }
             break;
 
-        default: oops();
+        default: ssassert(false, "Unexpected BSP insert type");
     }
     int i;
     for(i = 0; i < n - 2; i++) {
@@ -234,7 +234,7 @@ SBsp3 *SBsp3::InsertConvex(STriMeta meta, Vector *vertex, int cnt,
             inter[inters++] = vi;
         }
     }
-    if(npos > cnt + 1 || nneg > cnt + 1) oops();
+    if(npos > cnt + 1 || nneg > cnt + 1) ssassert(false, "Impossible");
 
     if(!instead) {
         if(inters == 2) {
@@ -320,7 +320,7 @@ void SBsp3::Insert(STriangle *tr, SMesh *instead) {
             if     (!isOn[0]) { a = tr->b; b = tr->c; }
             else if(!isOn[1]) { a = tr->c; b = tr->a; }
             else if(!isOn[2]) { a = tr->a; b = tr->b; }
-            else oops();
+            else ssassert(false, "Impossible");
             if(!instead) {
                 SEdge se = SEdge::From(a, b);
                 edges = SBsp2::InsertOrCreateEdge(edges, &se, n, tr->Normal());
@@ -344,7 +344,7 @@ void SBsp3::Insert(STriangle *tr, SMesh *instead) {
         if       (isOn[0]) { a = tr->a; b = tr->b; c = tr->c; bpos = isPos[1];
         } else if(isOn[1]) { a = tr->b; b = tr->c; c = tr->a; bpos = isPos[2];
         } else if(isOn[2]) { a = tr->c; b = tr->a; c = tr->b; bpos = isPos[0];
-        } else oops();
+        } else ssassert(false, "Impossible");
 
         Vector bPc = IntersectionWith(b, c);
         STriangle btri = STriangle::From(tr->meta, a, b, bPc);
@@ -371,14 +371,14 @@ void SBsp3::Insert(STriangle *tr, SMesh *instead) {
         if       (isNeg[0]) {   a = tr->a; b = tr->b; c = tr->c;
         } else if(isNeg[1]) {   a = tr->b; b = tr->c; c = tr->a;
         } else if(isNeg[2]) {   a = tr->c; b = tr->a; c = tr->b;
-        } else oops();
+        } else ssassert(false, "Impossible");
 
     } else if(posc == 1 && negc == 2) {
         if       (isPos[0]) {   a = tr->a; b = tr->b; c = tr->c;
         } else if(isPos[1]) {   a = tr->b; b = tr->c; c = tr->a;
         } else if(isPos[2]) {   a = tr->c; b = tr->a; c = tr->b;
-        } else oops();
-    } else oops();
+        } else ssassert(false, "Impossible");
+    } else ssassert(false, "Impossible");
 
     Vector aPb = IntersectionWith(a, b);
     Vector cPa = IntersectionWith(c, a);
@@ -471,7 +471,7 @@ void SBsp3::DebugDraw(void) {
 Vector SBsp2::IntersectionWith(Vector a, Vector b) {
     double da = a.Dot(no) - d;
     double db = b.Dot(no) - d;
-    if(da*db > 0) oops();
+    ssassert(da*db < 0, "Expected segment to intersect BSP node");
 
     double dab = (db - da);
     return (a.ScaledBy(db/dab)).Plus(b.ScaledBy(-da/dab));
@@ -547,7 +547,7 @@ void SBsp2::InsertEdge(SEdge *nedge, Vector nnp, Vector out) {
         }
         return;
     }
-    oops();
+    ssassert(false, "Impossible");
 }
 
 void SBsp2::InsertTriangleHow(int how, STriangle *tr, SMesh *m, SBsp3 *bsp3) {
@@ -568,7 +568,7 @@ void SBsp2::InsertTriangleHow(int how, STriangle *tr, SMesh *m, SBsp3 *bsp3) {
             }
             break;
 
-        default: oops();
+        default: ssassert(false, "Unexpected BSP insert type");
     }
 }
 
@@ -614,7 +614,7 @@ void SBsp2::InsertTriangle(STriangle *tr, SMesh *m, SBsp3 *bsp3) {
         if       (isOn[0]) { a = tr->a; b = tr->b; c = tr->c; bpos = isPos[1];
         } else if(isOn[1]) { a = tr->b; b = tr->c; c = tr->a; bpos = isPos[2];
         } else if(isOn[2]) { a = tr->c; b = tr->a; c = tr->b; bpos = isPos[0];
-        } else oops();
+        } else ssassert(false, "Impossible");
 
         Vector bPc = IntersectionWith(b, c);
         STriangle btri = STriangle::From(tr->meta, a, b, bPc);
@@ -636,14 +636,14 @@ void SBsp2::InsertTriangle(STriangle *tr, SMesh *m, SBsp3 *bsp3) {
         if       (isNeg[0]) {   a = tr->a; b = tr->b; c = tr->c;
         } else if(isNeg[1]) {   a = tr->b; b = tr->c; c = tr->a;
         } else if(isNeg[2]) {   a = tr->c; b = tr->a; c = tr->b;
-        } else oops();
+        } else ssassert(false, "Impossible");
 
     } else if(posc == 1 && negc == 2) {
         if       (isPos[0]) {   a = tr->a; b = tr->b; c = tr->c;
         } else if(isPos[1]) {   a = tr->b; b = tr->c; c = tr->a;
         } else if(isPos[2]) {   a = tr->c; b = tr->a; c = tr->b;
-        } else oops();
-    } else oops();
+        } else ssassert(false, "Impossible");
+    } else ssassert(false, "Impossible");
 
     Vector aPb = IntersectionWith(a, b);
     Vector cPa = IntersectionWith(c, a);
@@ -666,8 +666,8 @@ void SBsp2::InsertTriangle(STriangle *tr, SMesh *m, SBsp3 *bsp3) {
 }
 
 void SBsp2::DebugDraw(Vector n, double d) {
-    if(fabs((edge.a).Dot(n) - d) > LENGTH_EPS) oops();
-    if(fabs((edge.b).Dot(n) - d) > LENGTH_EPS) oops();
+    ssassert(fabs((edge.a).Dot(n) - d) < LENGTH_EPS, "Endpoint too close to BSP node plane");
+    ssassert(fabs((edge.b).Dot(n) - d) < LENGTH_EPS, "Endpoint too close to BSP node plane");
 
     ssglLineWidth(10);
     glBegin(GL_LINES);
diff --git a/src/clipboard.cpp b/src/clipboard.cpp
index a6be420..9055128 100644
--- a/src/clipboard.cpp
+++ b/src/clipboard.cpp
@@ -42,7 +42,8 @@ hEntity SolveSpaceUI::Clipboard::NewEntityFor(hEntity he) {
                 return cr->newReq.entity(1+i);
         }
     }
-    oops();
+
+    ssassert(false, "Expected to find entity in some clipboard request");
 }
 
 void GraphicsWindow::DeleteSelection(void) {
@@ -259,7 +260,7 @@ void GraphicsWindow::MenuClipboard(int id) {
             SS.GW.DeleteSelection();
             break;
 
-        default: oops();
+        default: ssassert(false, "Unexpected menu ID");
     }
 }
 
diff --git a/src/constraint.cpp b/src/constraint.cpp
index 6137021..5b70ddb 100644
--- a/src/constraint.cpp
+++ b/src/constraint.cpp
@@ -730,7 +730,7 @@ void Constraint::MenuConstrain(int id) {
             SS.ScheduleShowTW();
             break;
 
-        default: oops();
+        default: ssassert(false, "Unexpected menu ID");
     }
 
     SS.GW.ClearSelection();
diff --git a/src/constrainteq.cpp b/src/constrainteq.cpp
index 8acde19..69c9f0b 100644
--- a/src/constrainteq.cpp
+++ b/src/constrainteq.cpp
@@ -54,7 +54,7 @@ Expr *ConstraintBase::VectorsParallel(int eq, ExprVector a, ExprVector b) {
 
     if(eq == 0) return e0;
     if(eq == 1) return e1;
-    oops();
+    ssassert(false, "Unexpected index of equation");
 }
 
 Expr *ConstraintBase::PointLineDistance(hEntity wrkpl, hEntity hpt, hEntity hln)
@@ -104,7 +104,8 @@ Expr *ConstraintBase::PointPlaneDistance(ExprVector p, hEntity hpl) {
 Expr *ConstraintBase::Distance(hEntity wrkpl, hEntity hpa, hEntity hpb) {
     EntityBase *pa = SK.GetEntity(hpa);
     EntityBase *pb = SK.GetEntity(hpb);
-    if(!(pa->IsPoint() && pb->IsPoint())) oops();
+    ssassert(pa->IsPoint() && pb->IsPoint(),
+             "Expected two points to measure projected distance between");
 
     if(wrkpl.v == EntityBase::FREE_IN_3D.v) {
         // This is true distance
@@ -182,7 +183,7 @@ void ConstraintBase::ModifyToSatisfy(void) {
         IdList<Equation,hEquation> l = {};
         // Generate the equations even if this is a reference dimension
         GenerateReal(&l);
-        if(l.n != 1) oops();
+        ssassert(l.n == 1, "Expected constraint to generate a single equation");
 
         // These equations are written in the form f(...) - d = 0, where
         // d is the value of the valA.
@@ -735,7 +736,7 @@ void ConstraintBase::GenerateReal(IdList<Equation,hEquation> *l) {
                         dir[i] = e->CubicGetStartTangentExprs();
                     }
                 } else {
-                    oops();
+                    ssassert(false, "Unexpected entity types for CURVE_CURVE_TANGENT");
                 }
             }
             if(parallel) {
@@ -788,7 +789,7 @@ void ConstraintBase::GenerateReal(IdList<Equation,hEquation> *l) {
         case COMMENT:
             break;
 
-        default: oops();
+        default: ssassert(false, "Unexpected constraint ID");
     }
 }
 
diff --git a/src/describescreen.cpp b/src/describescreen.cpp
index b91993f..759142d 100644
--- a/src/describescreen.cpp
+++ b/src/describescreen.cpp
@@ -235,7 +235,7 @@ void TextWindow::DescribeSelection(void) {
             Printf(false, "%FtPOINT AND A CIRCLE");
         } else if(ec->type == Entity::ARC_OF_CIRCLE) {
             Printf(false, "%FtPOINT AND AN ARC");
-        } else oops();
+        } else ssassert(false, "Unexpected entity type");
         Vector p = SK.GetEntity(gs.point[0])->PointGetNum();
         Printf(true,  "        pt at " PT_AS_STR, COSTR(p));
         Vector c = SK.GetEntity(ec->point[0])->PointGetNum();
diff --git a/src/drawconstraint.cpp b/src/drawconstraint.cpp
index 31139bc..ecec403 100644
--- a/src/drawconstraint.cpp
+++ b/src/drawconstraint.cpp
@@ -305,7 +305,7 @@ void Constraint::DoEqualRadiusTicks(hEntity he) {
         double thetaa, thetab, dtheta;
         circ->ArcGetAngles(&thetaa, &thetab, &dtheta);
         theta = thetaa + dtheta/2;
-    } else oops();
+    } else ssassert(false, "Unexpected entity type");
 
     Vector d = u.ScaledBy(cos(theta)).Plus(v.ScaledBy(sin(theta)));
     d = d.ScaledBy(r);
@@ -1166,7 +1166,7 @@ s:
             break;
         }
 
-        default: oops();
+        default: ssassert(false, "Unexpected constraint type");
     }
 }
 
diff --git a/src/drawentity.cpp b/src/drawentity.cpp
index 39bc085..bfc7116 100644
--- a/src/drawentity.cpp
+++ b/src/drawentity.cpp
@@ -180,7 +180,7 @@ BBox Entity::GetOrGenerateScreenBBox(bool *hasBBox) {
                 screenBBox.Include(SS.GW.ProjectPoint3(sb->ctrl[i]));
             }
         }
-    } else oops();
+    } else ssassert(false, "Expected entity to be a point or have beziers");
 
     // Enlarge the bounding box to consider selection radius.
     screenBBox.minp.x -= SELECTION_RADIUS;
@@ -286,7 +286,7 @@ void Entity::ComputeInterpolatingSpline(SBezierList *sbl, bool periodic) {
 
     // The number of unknowns to solve for.
     int n   = periodic ? 3 + ep : ep;
-    if(n >= MAX_N) oops();
+    ssassert(n < MAX_N, "Too many unknowns");
     // The number of on-curve points, one more than the number of segments.
     int pts = periodic ? 4 + ep : 2 + ep;
 
@@ -708,8 +708,7 @@ void Entity::DrawOrGetDistance(void) {
             // Do nothing; these are drawn with the triangle mesh
             break;
 
-        default:
-            oops();
+        default: ssassert(false, "Unexpected entity type");
     }
 
     // And draw the curves; generate the rational polynomial curves for
diff --git a/src/dsc.h b/src/dsc.h
index 76d87b9..9336967 100644
--- a/src/dsc.h
+++ b/src/dsc.h
@@ -222,7 +222,7 @@ public:
     }
 
     void RemoveLast(int cnt) {
-        if(n < cnt) oops();
+        ssassert(n >= cnt, "Removing more elements than the list contains");
         for(int i = n - cnt; i < n; i++)
             elem[i].~T();
         n -= cnt;
@@ -281,13 +281,11 @@ public:
         while(first != last) {
             int mid = (first + last)/2;
             H hm = elem[mid].h;
+            ssassert(hm.v != t->h.v, "Handle isn't unique");
             if(hm.v > t->h.v) {
                 last = mid;
             } else if(hm.v < t->h.v) {
                 first = mid + 1;
-            } else {
-                dbp("can't insert in list; is handle %d not unique?", t->h.v);
-                oops();
             }
         }
 
@@ -300,10 +298,7 @@ public:
 
     T *FindById(H h) {
         T *t = FindByIdNoOops(h);
-        if(!t) {
-            dbp("failed to look up item %08x, searched %d items", h.v, n);
-            oops();
-        }
+        ssassert(t != NULL, "Cannot find handle");
         return t;
     }
 
diff --git a/src/entity.cpp b/src/entity.cpp
index 7f4a3a7..3c33522 100644
--- a/src/entity.cpp
+++ b/src/entity.cpp
@@ -38,7 +38,7 @@ ExprVector EntityBase::VectorGetExprs(void) {
         case NORMAL_N_ROT_AA:
             return NormalExprsN();
 
-        default: oops();
+        default: ssassert(false, "Unexpected entity type");
     }
 }
 
@@ -55,7 +55,7 @@ Vector EntityBase::VectorGetNum(void) {
         case NORMAL_N_ROT_AA:
             return NormalN();
 
-        default: oops();
+        default: ssassert(false, "Unexpected entity type");
     }
 }
 
@@ -72,7 +72,7 @@ Vector EntityBase::VectorGetRefPoint(void) {
         case NORMAL_N_ROT_AA:
             return SK.GetEntity(point[0])->PointGetNum();
 
-        default: oops();
+        default: ssassert(false, "Unexpected entity type");
     }
 }
 
@@ -88,7 +88,7 @@ Vector EntityBase::VectorGetStartPoint(void) {
         case NORMAL_N_ROT_AA:
             return SK.GetEntity(point[0])->PointGetNum();
 
-        default: oops();
+        default: ssassert(false, "Unexpected entity type");
     }
 }
 
@@ -101,7 +101,7 @@ Expr *EntityBase::CircleGetRadiusExpr(void) {
         return SK.GetEntity(distance)->DistanceGetExpr();
     } else if(type == ARC_OF_CIRCLE) {
         return Constraint::Distance(workplane, point[0], point[1]);
-    } else oops();
+    } else ssassert(false, "Unexpected entity type");
 }
 
 double EntityBase::CircleGetRadiusNum(void) {
@@ -111,11 +111,11 @@ double EntityBase::CircleGetRadiusNum(void) {
         Vector c  = SK.GetEntity(point[0])->PointGetNum();
         Vector pa = SK.GetEntity(point[1])->PointGetNum();
         return (pa.Minus(c)).Magnitude();
-    } else oops();
+    } else ssassert(false, "Unexpected entity type");
 }
 
 void EntityBase::ArcGetAngles(double *thetaa, double *thetab, double *dtheta) {
-    if(type != ARC_OF_CIRCLE) oops();
+    ssassert(type == ARC_OF_CIRCLE, "Unexpected entity type");
 
     Quaternion q = Normal()->NormalGetNum();
     Vector u = q.RotationU(), v = q.RotationV();
@@ -185,9 +185,7 @@ void EntityBase::WorkplaneGetPlaneExprs(ExprVector *n, Expr **dn) {
         //              n dot p - n dot p0 = 0
         // so dn = n dot p0
         *dn = p0.Dot(*n);
-    } else {
-        oops();
-    }
+    } else ssassert(false, "Unexpected entity type");
 }
 
 bool EntityBase::IsDistance(void) {
@@ -199,21 +197,21 @@ double EntityBase::DistanceGetNum(void) {
         return SK.GetParam(param[0])->val;
     } else if(type == DISTANCE_N_COPY) {
         return numDistance;
-    } else oops();
+    } else ssassert(false, "Unexpected entity type");
 }
 Expr *EntityBase::DistanceGetExpr(void) {
     if(type == DISTANCE) {
         return Expr::From(param[0]);
     } else if(type == DISTANCE_N_COPY) {
         return Expr::From(numDistance);
-    } else oops();
+    } else ssassert(false, "Unexpected entity type");
 }
 void EntityBase::DistanceForceTo(double v) {
     if(type == DISTANCE) {
         (SK.GetParam(param[0]))->val = v;
     } else if(type == DISTANCE_N_COPY) {
         // do nothing, it's locked
-    } else oops();
+    } else ssassert(false, "Unexpected entity type");
 }
 
 EntityBase *EntityBase::Normal(void) {
@@ -276,7 +274,7 @@ Quaternion EntityBase::NormalGetNum(void) {
             break;
         }
 
-        default: oops();
+        default: ssassert(false, "Unexpected entity type");
     }
     return q;
 }
@@ -308,7 +306,7 @@ void EntityBase::NormalForceTo(Quaternion q) {
             // Not sure if I'll bother implementing this one
             break;
 
-        default: oops();
+        default: ssassert(false, "Unexpected entity type");
     }
 }
 
@@ -364,7 +362,7 @@ ExprQuaternion EntityBase::NormalGetExprs(void) {
             break;
         }
 
-        default: oops();
+        default: ssassert(false, "Unexpected entity type");
     }
     return q;
 }
@@ -428,7 +426,7 @@ void EntityBase::PointForceTo(Vector p) {
             // Nothing to do; it's a static copy
             break;
 
-        default: oops();
+        default: ssassert(false, "Unexpected entity type");
     }
 }
 
@@ -476,7 +474,7 @@ Vector EntityBase::PointGetNum(void) {
             p = numPoint;
             break;
 
-        default: oops();
+        default: ssassert(false, "Unexpected entity type");
     }
     return p;
 }
@@ -525,7 +523,7 @@ ExprVector EntityBase::PointGetExprs(void) {
             r = ExprVector::From(numPoint);
             break;
 
-        default: oops();
+        default: ssassert(false, "Unexpected entity type");
     }
     return r;
 }
@@ -553,7 +551,7 @@ void EntityBase::PointGetExprsInWorkplane(hEntity wrkpl, Expr **u, Expr **v) {
 }
 
 void EntityBase::PointForceQuaternionTo(Quaternion q) {
-    if(type != POINT_N_ROT_TRANS) oops();
+    ssassert(type == POINT_N_ROT_TRANS, "Unexpected entity type");
 
     SK.GetParam(param[3])->val = q.w;
     SK.GetParam(param[4])->val = q.vx;
@@ -592,7 +590,7 @@ Quaternion EntityBase::PointGetQuaternion(void) {
         q = GetAxisAngleQuaternion(3);
     } else if(type == POINT_N_ROT_TRANS) {
         q = Quaternion::From(param[3], param[4], param[5], param[6]);
-    } else oops();
+    } else ssassert(false, "Unexpected entity type");
 
     return q;
 }
@@ -635,7 +633,7 @@ ExprVector EntityBase::FaceGetNormalExprs(void) {
         r = ExprVector::From(numNormal.vx, numNormal.vy, numNormal.vz);
         ExprQuaternion q = GetAxisAngleQuaternionExprs(3);
         r = q.Rotate(r);
-    } else oops();
+    } else ssassert(false, "Unexpected entity type");
     return r;
 }
 
@@ -658,7 +656,7 @@ Vector EntityBase::FaceGetNormalNum(void) {
         r = Vector::From(numNormal.vx, numNormal.vy, numNormal.vz);
         Quaternion q = GetAxisAngleQuaternion(3);
         r = q.Rotate(r);
-    } else oops();
+    } else ssassert(false, "Unexpected entity type");
     return r.WithMagnitude(1);
 }
 
@@ -687,7 +685,7 @@ ExprVector EntityBase::FaceGetPointExprs(void) {
         r = r.Minus(trans);
         r = q.Rotate(r);
         r = r.Plus(trans);
-    } else oops();
+    } else ssassert(false, "Unexpected entity type");
     return r;
 }
 
@@ -712,7 +710,7 @@ Vector EntityBase::FaceGetPointNum(void) {
         r = numPoint.Minus(trans);
         r = q.Rotate(r);
         r = r.Plus(trans);
-    } else oops();
+    } else ssassert(false, "Unexpected entity type");
     return r;
 }
 
@@ -728,9 +726,7 @@ Vector EntityBase::EndpointStart() {
         return CubicGetStartNum();
     } else if(type == ARC_OF_CIRCLE) {
         return SK.GetEntity(point[1])->PointGetNum();
-    } else {
-        oops();
-    }
+    } else ssassert(false, "Unexpected entity type");
 }
 Vector EntityBase::EndpointFinish() {
     if(type == LINE_SEGMENT) {
@@ -739,9 +735,7 @@ Vector EntityBase::EndpointFinish() {
         return CubicGetFinishNum();
     } else if(type == ARC_OF_CIRCLE) {
         return SK.GetEntity(point[2])->PointGetNum();
-    } else {
-        oops();
-    }
+    } else ssassert(false, "Unexpected entity type");
 }
 
 void EntityBase::AddEq(IdList<Equation,hEquation> *l, Expr *expr, int index) {
diff --git a/src/export.cpp b/src/export.cpp
index 8ceefd7..f38f0fc 100644
--- a/src/export.cpp
+++ b/src/export.cpp
@@ -895,7 +895,7 @@ void SolveSpaceUI::ExportMeshAsThreeJsTo(FILE *f, const std::string &filename,
 
     std::string baseFilename = noExtFilename;
     size_t lastSlash = baseFilename.rfind(PATH_SEP);
-    if(lastSlash == std::string::npos) oops();
+    ssassert(lastSlash != std::string::npos, "Expected at least one path separator");
     baseFilename.erase(0, lastSlash + 1);
 
     for(size_t i = 0; i < baseFilename.length(); i++) {
diff --git a/src/exportstep.cpp b/src/exportstep.cpp
index 8e3936c..5f6378c 100644
--- a/src/exportstep.cpp
+++ b/src/exportstep.cpp
@@ -117,7 +117,7 @@ int StepFileWriter::ExportCurve(SBezier *sb) {
 }
 
 int StepFileWriter::ExportCurveLoop(SBezierLoop *loop, bool inner) {
-    if(loop->l.n < 1) oops();
+    ssassert(loop->l.n >= 1, "Expected at least one loop");
 
     List<int> listOfTrims = {};
 
diff --git a/src/exportvector.cpp b/src/exportvector.cpp
index 3e274c0..b22b763 100644
--- a/src/exportvector.cpp
+++ b/src/exportvector.cpp
@@ -510,9 +510,7 @@ public:
             spline->knotslist.push_back(1.0);
             spline->knotslist.push_back(1.0);
             spline->knotslist.push_back(1.0);
-        } else {
-            oops();
-        }
+        } else ssassert(false, "Unexpected degree of spline");
     }
 
     void writeSpline(SBezier *sb) {
@@ -755,8 +753,7 @@ static std::string MakeStipplePattern(int pattern, double scale, char delimiter,
             result = ssprintf("%.3f_%.3f", scale * 2.0, scale * 0.5);
             break;
 
-        default:
-            oops();
+        default: ssassert(false, "Unexpected stipple pattern");
     }
     std::replace(result.begin(), result.end(), '_', delimiter);
     return result;
diff --git a/src/expr.cpp b/src/expr.cpp
index 52e06b0..f3c8b42 100644
--- a/src/expr.cpp
+++ b/src/expr.cpp
@@ -284,7 +284,7 @@ int Expr::Children(void) {
         case ACOS:
             return 1;
 
-        default: oops();
+        default: ssassert(false, "Unexpected operation");
     }
 }
 
@@ -293,7 +293,7 @@ int Expr::Nodes(void) {
         case 0: return 1;
         case 1: return 1 + a->Nodes();
         case 2: return 1 + a->Nodes() + b->Nodes();
-        default: oops();
+        default: ssassert(false, "Unexpected children count");
     }
 }
 
@@ -353,7 +353,7 @@ double Expr::Eval(void) {
         case ACOS:          return acos(a->Eval());
         case ASIN:          return asin(a->Eval());
 
-        default: oops();
+        default: ssassert(false, "Unexpected operation");
     }
 }
 
@@ -396,7 +396,7 @@ Expr *Expr::PartialWrt(hParam p) {
             return (From(-1)->Div((From(1)->Minus(a->Square()))->Sqrt()))
                         ->Times(a->PartialWrt(p));
 
-        default: oops();
+        default: ssassert(false, "Unexpected operation");
     }
 }
 
@@ -487,13 +487,13 @@ Expr *Expr::FoldConstants(void) {
             }
             break;
 
-        default: oops();
+        default: ssassert(false, "Unexpected operation");
     }
     return n;
 }
 
 void Expr::Substitute(hParam oldh, hParam newh) {
-    if(op == PARAM_PTR) oops();
+    ssassert(op != PARAM_PTR, "Expected an expression that refer to params via handles");
 
     if(op == PARAM && parh.v == oldh.v) {
         parh = newh;
@@ -518,7 +518,7 @@ hParam Expr::ReferencedParams(ParamList *pl) {
             return NO_PARAMS;
         }
     }
-    if(op == PARAM_PTR) oops();
+    ssassert(op != PARAM_PTR, "Expected an expression that refer to params via handles");
 
     int c = Children();
     if(c == 0) {
@@ -538,7 +538,7 @@ hParam Expr::ReferencedParams(ParamList *pl) {
         } else {
             return MULTIPLE_PARAMS;
         }
-    } else oops();
+    } else ssassert(false, "Unexpected children count");
 }
 
 
@@ -571,7 +571,7 @@ p:
         case ASIN:      return "(asin " + a->Print() + ")";
         case ACOS:      return "(acos " + a->Print() + ")";
 
-        default: oops();
+        default: ssassert(false, "Unexpected operation");
     }
 }
 
@@ -624,7 +624,7 @@ void Expr::Consume(void) {
 
 int Expr::Precedence(Expr *e) {
     if(e->op == ALL_RESOLVED) return -1; // never want to reduce this marker
-    if(e->op != BINARY_OP && e->op != UNARY_OP) oops();
+    ssassert(e->op == BINARY_OP || e->op == UNARY_OP, "Unexpected operation");
 
     switch(e->c) {
         case 'q':
@@ -638,7 +638,7 @@ int Expr::Precedence(Expr *e) {
         case '+':
         case '-':   return 10;
 
-        default: oops();
+        default: ssassert(false, "Unexpected operator");
     }
 }
 
@@ -664,7 +664,7 @@ c:
         case 's': n = (PopOperand()->Times(Expr::From(PI/180)))->Sin(); break;
         case 'c': n = (PopOperand()->Times(Expr::From(PI/180)))->Cos(); break;
 
-        default: oops();
+        default: ssassert(false, "Unexpected operator");
     }
     PushOperand(n);
 }
diff --git a/src/file.cpp b/src/file.cpp
index fe49590..705510f 100644
--- a/src/file.cpp
+++ b/src/file.cpp
@@ -248,7 +248,7 @@ void SolveSpaceUI::SaveUsingTable(int type) {
                 break;
             }
 
-            default: oops();
+            default: ssassert(false, "Unexpected value format");
         }
         fprintf(fh, "\n");
     }
@@ -415,7 +415,7 @@ void SolveSpaceUI::LoadUsingTable(char *key, char *val) {
                     break;
                 }
 
-                default: oops();
+                default: ssassert(false, "Unexpected value format");
             }
             break;
         }
@@ -569,7 +569,7 @@ bool SolveSpaceUI::LoadEntitiesFromFile(const std::string &filename, EntityList
                 &(tr.a.x), &(tr.a.y), &(tr.a.z),
                 &(tr.b.x), &(tr.b.y), &(tr.b.z),
                 &(tr.c.x), &(tr.c.y), &(tr.c.z)) != 11) {
-                oops();
+                ssassert(false, "Unexpected Triangle format");
             }
             tr.meta.color = RgbaColor::FromPackedInt((uint32_t)rgba);
             m->AddTriangle(&tr);
@@ -578,7 +578,7 @@ bool SolveSpaceUI::LoadEntitiesFromFile(const std::string &filename, EntityList
             if(sscanf(line, "Surface %x %x %x %d %d",
                 &(srf.h.v), &rgba, &(srf.face),
                 &(srf.degm), &(srf.degn)) != 5) {
-                oops();
+                ssassert(false, "Unexpected Surface format");
             }
             srf.color = RgbaColor::FromPackedInt((uint32_t)rgba);
         } else if(StrStartsWith(line, "SCtrl ")) {
@@ -588,7 +588,7 @@ bool SolveSpaceUI::LoadEntitiesFromFile(const std::string &filename, EntityList
             if(sscanf(line, "SCtrl %d %d %lf %lf %lf Weight %lf",
                                 &i, &j, &(c.x), &(c.y), &(c.z), &w) != 6)
             {
-                oops();
+                ssassert(false, "Unexpected SCtrl format");
             }
             srf.ctrl[i][j] = c;
             srf.weight[i][j] = w;
@@ -600,7 +600,7 @@ bool SolveSpaceUI::LoadEntitiesFromFile(const std::string &filename, EntityList
                 &(stb.start.x), &(stb.start.y), &(stb.start.z),
                 &(stb.finish.x), &(stb.finish.y), &(stb.finish.z)) != 8)
             {
-                oops();
+                ssassert(false, "Unexpected TrimBy format");
             }
             stb.backwards = (backwards != 0);
             srf.trim.Add(&stb);
@@ -615,7 +615,7 @@ bool SolveSpaceUI::LoadEntitiesFromFile(const std::string &filename, EntityList
                 &(crv.exact.deg),
                 &(crv.surfA.v), &(crv.surfB.v)) != 5)
             {
-                oops();
+                ssassert(false, "Unexpected Curve format");
             }
             crv.isExact = (isExact != 0);
         } else if(StrStartsWith(line, "CCtrl ")) {
@@ -625,7 +625,7 @@ bool SolveSpaceUI::LoadEntitiesFromFile(const std::string &filename, EntityList
             if(sscanf(line, "CCtrl %d %lf %lf %lf Weight %lf",
                                 &i, &(c.x), &(c.y), &(c.z), &w) != 5)
             {
-                oops();
+                ssassert(false, "Unexpected CCtrl format");
             }
             crv.exact.ctrl[i] = c;
             crv.exact.weight[i] = w;
@@ -636,16 +636,14 @@ bool SolveSpaceUI::LoadEntitiesFromFile(const std::string &filename, EntityList
                 &vertex,
                 &(scpt.p.x), &(scpt.p.y), &(scpt.p.z)) != 4)
             {
-                oops();
+                ssassert(false, "Unexpected CurvePt format");
             }
             scpt.vertex = (vertex != 0);
             crv.pts.Add(&scpt);
         } else if(strcmp(line, "AddCurve")==0) {
             sh->curve.Add(&crv);
             crv = {};
-        } else {
-            oops();
-        }
+        } else ssassert(false, "Unexpected operation");
     }
 
     fclose(fh);
@@ -733,7 +731,7 @@ static std::string MakePathAbsolute(const std::string &base, const std::string &
         if(part == ".") {
             /* do nothing */
         } else if(part == "..") {
-            if(resultParts.empty()) oops();
+            ssassert(!resultParts.empty(), "Relative path pointing outside of root directory");
             resultParts.pop_back();
         } else {
             resultParts.push_back(part);
diff --git a/src/generate.cpp b/src/generate.cpp
index 0931d7d..c96217c 100644
--- a/src/generate.cpp
+++ b/src/generate.cpp
@@ -101,7 +101,7 @@ bool SolveSpaceUI::PruneRequests(hGroup hg) {
 
         if(EntityExists(e->workplane)) continue;
 
-        if(!e->h.isFromRequest()) oops();
+        ssassert(e->h.isFromRequest(), "Only explicitly created entities can be pruned");
 
         (deleted.requests)++;
         SK.request.RemoveById(e->h.request());
@@ -199,7 +199,7 @@ void SolveSpaceUI::GenerateAll(GenerateType type, bool andFindFree, bool genForB
             break;
         }
 
-        default: oops();
+        default: ssassert(false, "Unexpected generation mode");
     }
 
     // If we're generating entities for display, first we need to find
diff --git a/src/glhelper.cpp b/src/glhelper.cpp
index e08aa07..9aacc3a 100644
--- a/src/glhelper.cpp
+++ b/src/glhelper.cpp
@@ -127,7 +127,7 @@ void ssglStippledLine(Vector a, Vector b, double width,
         case Style::STIPPLE_DOT:           stipplePattern = ".";   break;
         case Style::STIPPLE_FREEHAND:      stipplePattern = "~";   break;
         case Style::STIPPLE_ZIGZAG:        stipplePattern = "~__"; break;
-        default: oops();
+        default: ssassert(false, "Unexpected stipple pattern");
     }
     ssglStippledLine(a, b, width, stipplePattern, stippleScale, maybeFat);
 }
@@ -135,7 +135,7 @@ void ssglStippledLine(Vector a, Vector b, double width,
 void ssglStippledLine(Vector a, Vector b, double width,
                       const char *stipplePattern, double stippleScale, bool maybeFat)
 {
-    if(stipplePattern == NULL || *stipplePattern == 0) oops();
+    ssassert(stipplePattern != NULL, "Unexpected stipple pattern");
 
     Vector dir = b.Minus(a);
     double len = dir.Magnitude();
@@ -202,7 +202,7 @@ void ssglStippledLine(Vector a, Vector b, double width,
                 break;
             }
 
-            default: oops();
+            default: ssassert(false, "Unexpected stipple pattern element");
         }
         if(*(++si) == 0) si = stipplePattern;
     } while(end > 0.0);
diff --git a/src/graphicswin.cpp b/src/graphicswin.cpp
index 61fb4de..8776f29 100644
--- a/src/graphicswin.cpp
+++ b/src/graphicswin.cpp
@@ -596,7 +596,7 @@ void GraphicsWindow::MenuView(int id) {
             SS.GW.EnsureValidActives();
             break;
 
-        default: oops();
+        default: ssassert(false, "Unexpected menu ID");
     }
     InvalidateGraphics();
 }
@@ -938,7 +938,7 @@ void GraphicsWindow::MenuEdit(int id) {
             SS.ScheduleShowTW();
             break;
 
-        default: oops();
+        default: ssassert(false, "Unexpected menu ID");
     }
 }
 
@@ -1035,7 +1035,7 @@ c:
             SS.GW.SplitLinesOrCurves();
             break;
 
-        default: oops();
+        default: ssassert(false, "Unexpected menu ID");
     }
 }
 
diff --git a/src/group.cpp b/src/group.cpp
index 845cfff..2b921e5 100644
--- a/src/group.cpp
+++ b/src/group.cpp
@@ -241,7 +241,7 @@ void Group::MenuGroup(int id) {
             break;
         }
 
-        default: oops();
+        default: ssassert(false, "Unexpected menu ID");
     }
 
     // Copy color from the previous mesh-contributing group.
@@ -287,7 +287,7 @@ void Group::MenuGroup(int id) {
 }
 
 void Group::TransformImportedBy(Vector t, Quaternion q) {
-    if(type != LINKED) oops();
+    ssassert(type == LINKED, "Expected a linked group");
 
     hParam tx, ty, tz, qw, qx, qy, qz;
     tx = h.param(0);
@@ -362,7 +362,7 @@ void Group::Generate(IdList<Entity,hEntity> *entity,
             } else if(subtype == WORKPLANE_BY_POINT_ORTHO) {
                 // Already given, numerically.
                 q = predef.q;
-            } else oops();
+            } else ssassert(false, "Unexpected workplane subtype");
 
             Entity normal = {};
             normal.type = Entity::NORMAL_N_COPY;
@@ -398,7 +398,7 @@ void Group::Generate(IdList<Entity,hEntity> *entity,
                 ai = 0; af = 2;
             } else if(subtype == TWO_SIDED) {
                 ai = -1; af = 1;
-            } else oops();
+            } else ssassert(false, "Unexpected extrusion subtype");
 
             // Get some arbitrary point in the sketch, that will be used
             // as a reference when defining top and bottom faces.
@@ -549,7 +549,7 @@ void Group::Generate(IdList<Entity,hEntity> *entity,
             }
             break;
 
-        default: oops();
+        default: ssassert(false, "Unexpected group type");
     }
 }
 
diff --git a/src/importdxf.cpp b/src/importdxf.cpp
index a8b721d..2a698d0 100644
--- a/src/importdxf.cpp
+++ b/src/importdxf.cpp
@@ -685,7 +685,7 @@ public:
         if(addPendingBlockEntity<DRW_Insert>(data)) return;
 
         auto bi = blocks.find(data.name);
-        if(bi == blocks.end()) oops();
+        ssassert(bi != blocks.end(), "Inserted block does not exist");
         Block *block = &bi->second;
 
         // Push transform.
diff --git a/src/lib.cpp b/src/lib.cpp
index d87a4f0..d89370c 100644
--- a/src/lib.cpp
+++ b/src/lib.cpp
@@ -229,7 +229,7 @@ default: dbp("bad constraint type %d", sc->type); return;
             ssys->result = SLVS_RESULT_TOO_MANY_UNKNOWNS;
             break;
 
-        default: oops();
+        default: ssassert(false, "Unexpected solver result");
     }
 
     // Write the new parameter values back to our caller.
diff --git a/src/mesh.cpp b/src/mesh.cpp
index 4fec2b4..737a92e 100644
--- a/src/mesh.cpp
+++ b/src/mesh.cpp
@@ -1014,7 +1014,7 @@ void SKdNode::MakeCertainEdgesInto(SEdgeList *sel, int how, bool coplanarIsInter
                     }
                     break;
 
-                default: oops();
+                default: ssassert(false, "Unexpected edge class");
             }
 
             cnt++;
diff --git a/src/modify.cpp b/src/modify.cpp
index 47eee36..1d18fa0 100644
--- a/src/modify.cpp
+++ b/src/modify.cpp
@@ -131,9 +131,7 @@ void GraphicsWindow::ParametricCurve::MakeFromEntity(hEntity he, bool reverse) {
         EntityBase *wrkpln = SK.GetEntity(e->workplane)->Normal();
         u = wrkpln->NormalU();
         v = wrkpln->NormalV();
-    } else {
-        oops();
-    }
+    } else ssassert(false, "Unexpected entity type");
 }
 double GraphicsWindow::ParametricCurve::LengthForAuto(void) {
     if(isLine) {
@@ -518,7 +516,7 @@ hEntity GraphicsWindow::SplitCubic(hEntity he, Vector pinter) {
     int i, j;
     for(i = 0; i < sbl.l.n; i++) {
         SBezier *sb = &(sbl.l.elem[i]);
-        if(sb->deg != 3) oops();
+        ssassert(sb->deg == 3, "Expected a cubic bezier");
 
         sb->ClosestPointTo(pinter, &t, false);
         if(pinter.Equals(sb->PointAt(t))) {
diff --git a/src/mouse.cpp b/src/mouse.cpp
index a77e1e0..4da6218 100644
--- a/src/mouse.cpp
+++ b/src/mouse.cpp
@@ -455,7 +455,7 @@ void GraphicsWindow::MouseMoved(double x, double y, bool leftDown,
             InvalidateGraphics();
             break;
 
-        default: oops();
+        default: ssassert(false, "Unexpected pending operation");
     }
 
     if(pending.operation != 0 &&
@@ -590,7 +590,7 @@ void GraphicsWindow::MouseRightUp(double x, double y) {
                 Entity *e = SK.GetEntity(gs.entity[0]);
                 e->GetDistance(Point2d::From(x, y));
                 addAfterPoint = e->dogd.data;
-                if(addAfterPoint == -1) oops();
+                ssassert(addAfterPoint != -1, "Expected a nearest bezier point to be located");
                 // Skip derivative point.
                 if(r->type == Request::CUBIC) addAfterPoint++;
                 AddContextMenuItem("Add Spline Point", CMNU_ADD_SPLINE_PT);
@@ -708,7 +708,7 @@ void GraphicsWindow::MouseRightUp(double x, double y) {
             Request *r = SK.GetRequest(hr);
 
             int index = r->IndexOfPoint(gs.point[0]);
-            if(r->extraPoints == 0) oops();
+            ssassert(r->extraPoints != 0, "Expected a bezier with interior control points");
 
             SS.UndoRemember();
             Entity *e = SK.GetEntity(r->h.entity(0));
diff --git a/src/platform/cocoamain.mm b/src/platform/cocoamain.mm
index 7fd2a6e..069923b 100644
--- a/src/platform/cocoamain.mm
+++ b/src/platform/cocoamain.mm
@@ -409,7 +409,7 @@ CONVERT(Rect)
 - (void)prepareEditorWithMinWidthInChars:(int)minWidthChars {
     NSFont *font = [editor font];
     NSGlyph glyphA = [font glyphWithName:@"a"];
-    if(glyphA == -1) oops();
+    ssassert(glyphA != -1, "Expected font to have U+0061");
     CGFloat glyphAWidth = [font advancementForGlyph:glyphA].width;
 
     [editor sizeToFit];
@@ -578,7 +578,7 @@ void AddContextMenuItem(const char *label, int id_) {
 }
 
 void CreateContextSubmenu(void) {
-    if(contextSubmenu) oops();
+    ssassert(!contextSubmenu, "Unexpected nested submenu");
 
     contextSubmenu = [[NSMenu alloc] initWithTitle:@""];
 }
@@ -637,8 +637,8 @@ void InitMainMenu(NSMenu *mainMenu) {
             [menu setAutoenablesItems:NO];
             [menuItem setSubmenu:menu];
 
-            if(entry->level >= sizeof(levels) / sizeof(levels[0]))
-                oops();
+            ssassert((unsigned)entry->level < sizeof(levels) / sizeof(levels[0]),
+                     "Unexpected depth of menu nesting");
 
             levels[entry->level] = menu;
         }
@@ -1145,7 +1145,7 @@ const void *SolveSpace::LoadResource(const std::string &name, size_t *size) {
     if(data == nil) {
         NSString *path = [[NSBundle mainBundle] pathForResource:key ofType:nil];
         data = [[NSFileHandle fileHandleForReadingAtPath:path] readDataToEndOfFile];
-        if(data == nil) oops();
+        ssassert(data != nil, "Cannot find resource");
         [cache setObject:data forKey:key];
     }
 
diff --git a/src/platform/gloffscreen.cpp b/src/platform/gloffscreen.cpp
index be836e2..84ee0e2 100644
--- a/src/platform/gloffscreen.cpp
+++ b/src/platform/gloffscreen.cpp
@@ -14,12 +14,10 @@
 
 GLOffscreen::GLOffscreen() : _pixels(NULL), _pixels_inv(NULL), _width(0), _height(0) {
 #ifndef __APPLE__
-    if(glewInit() != GLEW_OK)
-        oops();
+    ssassert(glewInit() == GLEW_OK, "Unexpected GLEW failure");
 #endif
 
-    if(!GL_EXT_framebuffer_object)
-        oops();
+    ssassert(GL_EXT_framebuffer_object, "Expected an available FBO extension");
 
     glGenFramebuffersEXT(1, &_framebuffer);
     glGenRenderbuffersEXT(1, &_color_renderbuffer);
diff --git a/src/platform/gtkmain.cpp b/src/platform/gtkmain.cpp
index 077ee7f..90a47a7 100644
--- a/src/platform/gtkmain.cpp
+++ b/src/platform/gtkmain.cpp
@@ -243,15 +243,11 @@ public:
         _xdisplay = gdk_x11_get_default_xdisplay();
 
         int glxmajor, glxminor;
-        if(!glXQueryVersion(_xdisplay, &glxmajor, &glxminor)) {
-            dbp("OpenGL is not supported");
-            oops();
-        }
+        ssassert(glXQueryVersion(_xdisplay, &glxmajor, &glxminor),
+                 "Expected OpenGL to be available");
 
-        if(glxmajor < 1 || (glxmajor == 1 && glxminor < 3)) {
-            dbp("GLX version %d.%d is too old; 1.3 required", glxmajor, glxminor);
-            oops();
-        }
+        ssassert(glxmajor > 1 || (glxmajor == 1 && glxminor >= 3),
+                 "Expected GLX >= 1.3");
 
         static int fbconfig_attrs[] = {
             GLX_RENDER_TYPE, GLX_RGBA_BIT,
@@ -264,8 +260,8 @@ public:
         int fbconfig_num = 0;
         GLXFBConfig *fbconfigs = glXChooseFBConfig(_xdisplay, DefaultScreen(_xdisplay),
                 fbconfig_attrs, &fbconfig_num);
-        if(!fbconfigs || fbconfig_num == 0)
-            oops();
+        ssassert(fbconfigs && fbconfig_num > 0,
+                 "Expected an available framebuffer configuration");
 
         /* prefer FBConfigs with depth of 32;
             * Mesa software rasterizer explodes with a BadMatch without this;
@@ -291,10 +287,7 @@ public:
 
         _glcontext = glXCreateNewContext(_xdisplay,
                 fbconfig, GLX_RGBA_TYPE, 0, True);
-        if(!_glcontext) {
-            dbp("cannot create OpenGL context");
-            oops();
-        }
+        ssassert(_glcontext != NULL, "Cannot create an OpenGL context");
 
         XFree(fbconfigs);
 
@@ -329,15 +322,15 @@ protected:
     bool on_expose_event(GdkEventExpose *) override {
         const Cairo::RefPtr<Cairo::Context> &cr = get_window()->create_cairo_context();
 #endif
-        if(!glXMakeCurrent(_xdisplay, _xwindow, _glcontext))
-            oops();
+        ssassert(glXMakeCurrent(_xdisplay, _xwindow, _glcontext),
+                 "Cannot make OpenGL context current");
 
         if(!_offscreen)
             _offscreen = new GLOffscreen;
 
         Gdk::Rectangle allocation = get_allocation();
-        if(!_offscreen->begin(allocation.get_width(), allocation.get_height()))
-            oops();
+        ssassert(_offscreen->begin(allocation.get_width(), allocation.get_height()),
+                 "Cannot allocate offscreen rendering buffer");
 
         on_gl_draw();
         glFlush();
@@ -860,7 +853,7 @@ void AddContextMenuItem(const char *label, int id) {
 }
 
 void CreateContextSubmenu(void) {
-    if(context_submenu) oops();
+    ssassert(!context_submenu, "Unexpected nested submenu");
 
     context_submenu = new Gtk::Menu;
 }
@@ -969,8 +962,8 @@ static void InitMainMenu(Gtk::MenuShell *menu_shell) {
             Gtk::Menu *menu = new Gtk::Menu;
             menu_item->set_submenu(*menu);
 
-            if((unsigned)entry->level >= sizeof(levels) / sizeof(levels[0]))
-                oops();
+            ssassert((unsigned)entry->level < sizeof(levels) / sizeof(levels[0]),
+                     "Unexpected depth of menu nesting");
 
             levels[entry->level] = menu;
         }
@@ -1144,8 +1137,7 @@ static void ChooserFilterChanged(Gtk::FileChooserDialog *chooser)
     int rdelim = filter_name.find(',', lparen);
     if(rdelim < 0)
         rdelim = filter_name.find(')', lparen);
-    if(lparen < 0 || rdelim < 0)
-        oops();
+    ssassert(lparen > 0 && rdelim > 0, "Expected to find a parenthesized extension");
 
     std::string extension = filter_name.substr(lparen, rdelim - lparen);
     if(extension == "*")
@@ -1506,14 +1498,14 @@ const void *LoadResource(const std::string &name, size_t *size) {
 
         path = (UNIX_DATADIR "/") + name;
         if(stat(path.c_str(), &st)) {
-            if(errno != ENOENT) oops();
+            ssassert(errno == ENOENT, "Unexpected stat() error");
             path = resource_dir + "/" + name;
-            if(stat(path.c_str(), &st)) oops();
+            ssassert(!stat(path.c_str(), &st), "Cannot find resource");
         }
 
         std::vector<uint8_t> data(st.st_size);
         FILE *f = ssfopen(path.c_str(), "rb");
-        if(!f) oops();
+        ssassert(f != NULL, "Cannot open resource");
         fread(&data[0], 1, st.st_size, f);
         fclose(f);
 
diff --git a/src/platform/unixutil.cpp b/src/platform/unixutil.cpp
index ef1a86a..31366dd 100644
--- a/src/platform/unixutil.cpp
+++ b/src/platform/unixutil.cpp
@@ -51,13 +51,15 @@ void assert_failure(const char *file, unsigned line, const char *function,
 
 FILE *ssfopen(const std::string &filename, const char *mode)
 {
-    if(filename.length() != strlen(filename.c_str())) oops();
+    ssassert(filename.length() == strlen(filename.c_str()),
+             "Unexpected null byte in middle of a path");
     return fopen(filename.c_str(), mode);
 }
 
 void ssremove(const std::string &filename)
 {
-    if(filename.length() != strlen(filename.c_str())) oops();
+    ssassert(filename.length() == strlen(filename.c_str()),
+             "Unexpected null byte in middle of a path");
     remove(filename.c_str());
 }
 
@@ -114,7 +116,7 @@ void FreeAllTemporary(void)
 
 void *MemAlloc(size_t n) {
     void *p = malloc(n);
-    if(!p) oops();
+    ssassert(p != NULL, "Cannot allocate memory");
     return p;
 }
 
diff --git a/src/platform/w32main.cpp b/src/platform/w32main.cpp
index 72b3316..a3fe02b 100644
--- a/src/platform/w32main.cpp
+++ b/src/platform/w32main.cpp
@@ -744,9 +744,9 @@ static void CreateGlContext(HWND hwnd, HGLRC *glrc)
     pfd.cStencilBits = 0;
 
     pixelFormat = ChoosePixelFormat(hdc, &pfd);
-    if(!pixelFormat) oops();
+    ssassert(pixelFormat != 0, "Expected a valid pixel format to be chosen");
 
-    if(!SetPixelFormat(hdc, pixelFormat, &pfd)) oops();
+    ssassert(SetPixelFormat(hdc, pixelFormat, &pfd), "Cannot set pixel format");
 
     *glrc = wglCreateContext(hdc);
     wglMakeCurrent(hdc, *glrc);
@@ -928,8 +928,6 @@ LRESULT CALLBACK GraphicsWndProc(HWND hwnd, UINT msg, WPARAM wParam,
                     !!(wParam & MK_RBUTTON),
                     !!(wParam & MK_SHIFT),
                     !!(wParam & MK_CONTROL));
-            } else {
-                oops();
             }
             break;
         }
@@ -955,7 +953,7 @@ LRESULT CALLBACK GraphicsWndProc(HWND hwnd, UINT msg, WPARAM wParam,
                         break;
                     }
                 }
-                if(SS.GW.menu[i].level < 0) oops();
+                ssassert(SS.GW.menu[i].level >= 0, "Cannot find command in the menu");
             }
             break;
         }
@@ -1153,8 +1151,8 @@ static void MenuById(int id, bool yes, bool check)
         if(SS.GW.menu[i].level == 0) subMenu++;
 
         if(SS.GW.menu[i].id == id) {
-            if(subMenu < 0) oops();
-            if(subMenu >= (int)arraylen(SubMenus)) oops();
+            ssassert(subMenu >= 0 && subMenu < (int)arraylen(SubMenus),
+                     "Submenu out of range");
 
             if(check) {
                 CheckMenuItem(SubMenus[subMenu], id,
@@ -1166,7 +1164,7 @@ static void MenuById(int id, bool yes, bool check)
             return;
         }
     }
-    oops();
+    ssassert(false, "Cannot find submenu");
 }
 void SolveSpace::CheckMenuById(int id, bool checked)
 {
@@ -1219,7 +1217,7 @@ HMENU CreateGraphicsWindowMenus(void)
         if(SS.GW.menu[i].level == 0) {
             m = CreateMenu();
             AppendMenuW(top, MF_STRING | MF_POPUP, (UINT_PTR)m, Widen(label).c_str());
-            if(subMenu >= (int)arraylen(SubMenus)) oops();
+            ssassert(subMenu < (int)arraylen(SubMenus), "Too many submenus");
             SubMenus[subMenu] = m;
             subMenu++;
         } else if(SS.GW.menu[i].level == 1) {
@@ -1236,7 +1234,7 @@ HMENU CreateGraphicsWindowMenus(void)
             } else {
                 AppendMenuW(m, MF_SEPARATOR, SS.GW.menu[i].id, L"");
             }
-        } else oops();
+        } else ssassert(false, "Submenus nested too deeply");
     }
     RefreshRecentMenus();
 
@@ -1261,7 +1259,7 @@ static void CreateMainWindows(void)
                             IMAGE_ICON, 32, 32, 0);
     wc.hIconSm          = (HICON)LoadImage(Instance, MAKEINTRESOURCE(4000),
                             IMAGE_ICON, 16, 16, 0);
-    if(!RegisterClassEx(&wc)) oops();
+    ssassert(RegisterClassEx(&wc), "Cannot register window class");
 
     HMENU top = CreateGraphicsWindowMenus();
     GraphicsWnd = CreateWindowExW(0, L"GraphicsWnd",
@@ -1269,7 +1267,7 @@ static void CreateMainWindows(void)
         WS_OVERLAPPED | WS_THICKFRAME | WS_CLIPCHILDREN | WS_MAXIMIZEBOX |
         WS_MINIMIZEBOX | WS_SYSMENU | WS_SIZEBOX | WS_CLIPSIBLINGS,
         50, 50, 900, 600, NULL, top, Instance, NULL);
-    if(!GraphicsWnd) oops();
+    ssassert(GraphicsWnd != NULL, "Cannot create window");
 
     GraphicsEditControl = CreateWindowExW(WS_EX_CLIENTEDGE, WC_EDIT, L"",
         WS_CHILD | ES_AUTOHSCROLL | WS_TABSTOP | WS_CLIPSIBLINGS,
@@ -1282,14 +1280,14 @@ static void CreateMainWindows(void)
     wc.hbrBackground    = (HBRUSH)GetStockObject(BLACK_BRUSH);
     wc.lpszClassName    = L"TextWnd";
     wc.hCursor          = NULL;
-    if(!RegisterClassEx(&wc)) oops();
+    ssassert(RegisterClassEx(&wc), "Cannot register window class");
 
     // We get the desired Alt+Tab behaviour by specifying that the text
     // window is a child of the graphics window.
     TextWnd = CreateWindowExW(0,
         L"TextWnd", L"SolveSpace - Property Browser", WS_THICKFRAME | WS_CLIPCHILDREN,
         650, 500, 420, 300, GraphicsWnd, (HMENU)NULL, Instance, NULL);
-    if(!TextWnd) oops();
+    ssassert(TextWnd != NULL, "Cannot create window");
 
     TextWndScrollBar = CreateWindowExW(0, WC_SCROLLBAR, L"", WS_CHILD |
         SBS_VERT | SBS_LEFTALIGN | WS_VISIBLE | WS_CLIPSIBLINGS,
@@ -1313,9 +1311,9 @@ static void CreateMainWindows(void)
 
 const void *SolveSpace::LoadResource(const std::string &name, size_t *size) {
     HRSRC hres = FindResourceW(NULL, Widen(name).c_str(), RT_RCDATA);
-    if(!hres) oops();
+    ssassert(hres != NULL, "Cannot find resource");
     HGLOBAL res = LoadResource(NULL, hres);
-    if(!res) oops();
+    ssassert(res != NULL, "Cannot load resource");
 
     *size = SizeofResource(NULL, hres);
     return LockResource(res);
diff --git a/src/platform/w32util.cpp b/src/platform/w32util.cpp
index 588e04d..b9d72a8 100644
--- a/src/platform/w32util.cpp
+++ b/src/platform/w32util.cpp
@@ -40,8 +40,8 @@ std::string Narrow(const wchar_t *in)
     std::string out;
     DWORD len = WideCharToMultiByte(CP_UTF8, 0, in, -1, NULL, 0, NULL, NULL);
     out.resize(len - 1);
-    if(!WideCharToMultiByte(CP_UTF8, 0, in, -1, &out[0], len, NULL, NULL))
-        oops();
+    ssassert(WideCharToMultiByte(CP_UTF8, 0, in, -1, &out[0], len, NULL, NULL),
+             "Invalid UTF-16");
     return out;
 }
 
@@ -51,9 +51,9 @@ std::string Narrow(const std::wstring &in)
 
     std::string out;
     out.resize(WideCharToMultiByte(CP_UTF8, 0, &in[0], in.length(), NULL, 0, NULL, NULL));
-    if(!WideCharToMultiByte(CP_UTF8, 0, &in[0], in.length(),
-                            &out[0], out.length(), NULL, NULL))
-        oops();
+    ssassert(WideCharToMultiByte(CP_UTF8, 0, &in[0], in.length(),
+                                 &out[0], out.length(), NULL, NULL),
+             "Invalid UTF-16");
     return out;
 }
 
@@ -62,8 +62,8 @@ std::wstring Widen(const char *in)
     std::wstring out;
     DWORD len = MultiByteToWideChar(CP_UTF8, 0, in, -1, NULL, 0);
     out.resize(len - 1);
-    if(!MultiByteToWideChar(CP_UTF8, 0, in, -1, &out[0], len))
-        oops();
+    ssassert(MultiByteToWideChar(CP_UTF8, 0, in, -1, &out[0], len),
+             "Invalid UTF-8");
     return out;
 }
 
@@ -73,8 +73,8 @@ std::wstring Widen(const std::string &in)
 
     std::wstring out;
     out.resize(MultiByteToWideChar(CP_UTF8, 0, &in[0], in.length(), NULL, 0));
-    if(!MultiByteToWideChar(CP_UTF8, 0, &in[0], in.length(), &out[0], out.length()))
-        oops();
+    ssassert(MultiByteToWideChar(CP_UTF8, 0, &in[0], in.length(), &out[0], out.length()),
+             "Invalid UTF-8");
     return out;
 }
 
@@ -88,13 +88,15 @@ FILE *ssfopen(const std::string &filename, const char *mode)
     if(uncFilename.substr(0, 2) != "\\\\")
         uncFilename = "\\\\?\\" + uncFilename;
 
-    if(filename.length() != strlen(filename.c_str())) oops();
+    ssassert(filename.length() == strlen(filename.c_str()),
+             "Unexpected null byte in middle of a path");
     return _wfopen(Widen(uncFilename).c_str(), Widen(mode).c_str());
 }
 
 void ssremove(const std::string &filename)
 {
-    if(filename.length() != strlen(filename.c_str())) oops();
+    ssassert(filename.length() == strlen(filename.c_str()),
+             "Unexpected null byte in middle of a path");
     _wremove(Widen(filename).c_str());
 }
 
@@ -107,7 +109,7 @@ void ssremove(const std::string &filename)
 void *AllocTemporary(size_t n)
 {
     void *v = HeapAlloc(TempHeap, HEAP_NO_SERIALIZE | HEAP_ZERO_MEMORY, n);
-    if(!v) oops();
+    ssassert(v != NULL, "Cannot allocate memory");
     return v;
 }
 void FreeTemporary(void *p) {
@@ -124,7 +126,7 @@ void FreeAllTemporary(void)
 
 void *MemAlloc(size_t n) {
     void *p = HeapAlloc(PermHeap, HEAP_NO_SERIALIZE | HEAP_ZERO_MEMORY, n);
-    if(!p) oops();
+    ssassert(p != NULL, "Cannot allocate memory");
     return p;
 }
 void MemFree(void *p) {
@@ -132,8 +134,8 @@ void MemFree(void *p) {
 }
 
 void vl(void) {
-    if(!HeapValidate(TempHeap, HEAP_NO_SERIALIZE, NULL)) oops();
-    if(!HeapValidate(PermHeap, HEAP_NO_SERIALIZE, NULL)) oops();
+    ssassert(HeapValidate(TempHeap, HEAP_NO_SERIALIZE, NULL), "Corrupted heap");
+    ssassert(HeapValidate(PermHeap, HEAP_NO_SERIALIZE, NULL), "Corrupted heap");
 }
 
 void InitHeaps(void) {
diff --git a/src/polygon.cpp b/src/polygon.cpp
index eae40fe..bc1f322 100644
--- a/src/polygon.cpp
+++ b/src/polygon.cpp
@@ -545,7 +545,7 @@ Vector SContour::ComputeNormal(void) {
 }
 
 Vector SContour::AnyEdgeMidpoint(void) {
-    if(l.n < 2) oops();
+    ssassert(l.n >= 2, "Need two points to find a midpoint");
     return ((l.elem[0].p).Plus(l.elem[1].p)).ScaledBy(0.5);
 }
 
@@ -696,7 +696,7 @@ bool SPolygon::IsEmpty(void) {
 }
 
 Vector SPolygon::AnyPoint(void) {
-    if(IsEmpty()) oops();
+    ssassert(!IsEmpty(), "Need at least one point");
     return l.elem[0].l.elem[0].p;
 }
 
diff --git a/src/request.cpp b/src/request.cpp
index b770e06..03db380 100644
--- a/src/request.cpp
+++ b/src/request.cpp
@@ -147,7 +147,7 @@ void Request::Generate(IdList<Entity,hEntity> *entity,
             // and this is just a copy of the workplane quaternion,
             // so no params required
         }
-        if(points < 1) oops();
+        ssassert(points >= 1, "Positioning a normal requires a point");
         // The point determines where the normal gets displayed on-screen;
         // it's entirely cosmetic.
         n.point[0] = e.point[0];
diff --git a/src/resource.cpp b/src/resource.cpp
index 808a7d5..5f590c1 100644
--- a/src/resource.cpp
+++ b/src/resource.cpp
@@ -17,36 +17,31 @@ namespace SolveSpace {
 std::string LoadString(const std::string &name) {
     size_t size;
     const void *data = LoadResource(name, &size);
-    if(data == NULL) oops();
-
     return std::string(static_cast<const char *>(data), size);
 }
 
 std::string LoadStringFromGzip(const std::string &name) {
     size_t size;
     const void *data = LoadResource(name, &size);
-    if(data == NULL) oops();
 
     z_stream stream;
     stream.zalloc = Z_NULL;
     stream.zfree = Z_NULL;
     stream.opaque = Z_NULL;
-    if(inflateInit2(&stream, /*decode gzip header*/16) != Z_OK)
-        oops();
+    ssassert(inflateInit2(&stream, /*decode gzip header*/16) == Z_OK,
+             "Cannot start inflation");
 
     // Extract length mod 2**32 from the gzip trailer.
     std::string result;
-    if(size < 4) oops();
+    ssassert(size >= 4, "Resource too small to have gzip trailer");
     result.resize(*(uint32_t *)((uintptr_t)data + size - 4));
 
     stream.next_in = (Bytef *)data;
     stream.avail_in = size;
     stream.next_out = (Bytef *)&result[0];
     stream.avail_out = result.length();
-    if(inflate(&stream, Z_NO_FLUSH) != Z_STREAM_END)
-        oops();
-    if(stream.avail_out != 0)
-        oops();
+    ssassert(inflate(&stream, Z_NO_FLUSH) == Z_STREAM_END, "Cannot inflate resource");
+    ssassert(stream.avail_out == 0, "Inflated resource larger than what trailer indicates");
 
     inflateEnd(&stream);
 
@@ -56,10 +51,9 @@ std::string LoadStringFromGzip(const std::string &name) {
 Pixmap LoadPNG(const std::string &name) {
     size_t size;
     const void *data = LoadResource(name, &size);
-    if(data == NULL) oops();
 
     Pixmap pixmap = Pixmap::FromPNG(static_cast<const uint8_t *>(data), size);
-    if(pixmap.IsEmpty()) oops();
+    ssassert(!pixmap.IsEmpty(), "Cannot load pixmap");
 
     return pixmap;
 }
@@ -189,12 +183,12 @@ public:
     }
 
     char ReadChar() {
-        if(AtEnd()) oops();
+        ssassert(!AtEnd(), "Unexpected EOF");
         return *pos++;
     }
 
     bool TryChar(char c) {
-        if(AtEnd()) oops();
+        ssassert(!AtEnd(), "Unexpected EOF");
         if(*pos == c) {
             pos++;
             return true;
@@ -204,7 +198,7 @@ public:
     }
 
     void ExpectChar(char c) {
-        if(ReadChar() != c) oops();
+        ssassert(ReadChar() == c, "Unexpected character");
     }
 
     uint8_t Read4HexBits() {
@@ -215,7 +209,7 @@ public:
             return 10 + (c - 'a');
         } else if(c >= 'A' && c <= 'F') {
             return 10 + (c - 'A');
-        } else oops();
+        } else ssassert(false, "Unexpected hex digit");
     }
 
     uint8_t Read8HexBits() {
@@ -233,12 +227,12 @@ public:
     double ReadDoubleString() {
         char *endptr;
         double d = strtod(&*pos, &endptr);
-        if(&*pos == endptr) oops();
+        ssassert(&*pos != endptr, "Cannot read a double-precision number");
         pos += endptr - &*pos;
         return d;
     }
 
-    bool ReadRegex(const std::regex &re, std::smatch *m) {
+    bool TryRegex(const std::regex &re, std::smatch *m) {
         if(std::regex_search(pos, end, *m, re, std::regex_constants::match_continuous)) {
             pos = (*m)[0].second;
             return true;
@@ -269,9 +263,12 @@ BitmapFont BitmapFont::From(std::string &&unifontData) {
 }
 
 void BitmapFont::AddGlyph(char32_t codepoint, const Pixmap &pixmap) {
-    if(pixmap.width != 8 && pixmap.width != 16 && pixmap.height != 16) oops();
-    if(glyphs.find(codepoint) != glyphs.end()) oops();
-    if(nextPosition == 0xffff) oops();
+    ssassert((pixmap.width == 8 || pixmap.width == 16) && pixmap.height == 16,
+             "Unexpected glyph dimensions");
+    ssassert(glyphs.find(codepoint) == glyphs.end(),
+             "Glyph with this codepoint already exists");
+    ssassert(nextPosition != 0xffff,
+             "Too many glyphs for current texture size");
 
     BitmapFont::Glyph glyph = {};
     glyph.advanceCells = pixmap.width / 8;
@@ -294,7 +291,8 @@ const BitmapFont::Glyph &BitmapFont::GetGlyph(char32_t codepoint) {
         return (*it).second;
     }
 
-    if(nextPosition == 0xffff) oops();
+    ssassert(nextPosition != 0xffff,
+             "Too many glyphs for current texture size");
 
     // Find the hex representation in the (sorted) Unifont file.
     auto first = unifontData.cbegin(),
@@ -344,7 +342,7 @@ const BitmapFont::Glyph &BitmapFont::GetGlyph(char32_t codepoint) {
             for(size_t i = 0; i < 16; i++) {
                 glyphBits[i] = (uint16_t)reader.Read8HexBits() << 8;
             }
-        } else oops();
+        } else ssassert(false, "Unexpected glyph bitmap length");
 
         // Fill in the texture (one texture byte per glyph bit).
         for(size_t y = 0; y < 16; y++) {
@@ -361,7 +359,7 @@ const BitmapFont::Glyph &BitmapFont::GetGlyph(char32_t codepoint) {
     }
 
     // Glyph doesn't exist; return replacement glyph instead.
-    if(codepoint == 0xfffd) oops();
+    ssassert(codepoint != 0xfffd, "Cannot parse replacement glyph");
     return GetGlyph(0xfffd);
 }
 
@@ -461,7 +459,7 @@ VectorFont VectorFont::From(std::string &&lffData) {
 
     ASCIIReader reader = ASCIIReader::From(font.lffData);
     std::smatch m;
-    while(reader.ReadRegex(std::regex("#\\s*(\\w+)\\s*:\\s*(.+?)\n"), &m)) {
+    while(reader.TryRegex(std::regex("#\\s*(\\w+)\\s*:\\s*(.+?)\n"), &m)) {
         std::string name  = m.str(1),
                     value = m.str(2);
         std::transform(name.begin(), name.end(), name.begin(), ::tolower);
@@ -488,7 +486,7 @@ const VectorFont::Glyph &VectorFont::GetGlyph(char32_t codepoint) {
     }
 
     auto firstGlyph = std::find(lffData.cbegin(), lffData.cend(), '[');
-    if(firstGlyph == lffData.cend()) oops();
+    ssassert(firstGlyph != lffData.cend(), "Vector font contains no glyphs");
 
     // Find the serialized representation in the (sorted) lff file.
     auto first = firstGlyph,
@@ -572,7 +570,7 @@ const VectorFont::Glyph &VectorFont::GetGlyph(char32_t codepoint) {
     }
 
     // Glyph doesn't exist; return replacement glyph instead.
-    if(codepoint == 0xfffd) oops();
+    ssassert(codepoint != 0xfffd, "Cannot parse replacement glyph");
     return GetGlyph(0xfffd);
 }
 
diff --git a/src/sketch.h b/src/sketch.h
index 26625fe..18189ba 100644
--- a/src/sketch.h
+++ b/src/sketch.h
@@ -881,7 +881,7 @@ inline hRequest hEntity::request(void)
 inline hGroup hEntity::group(void)
     { hGroup r; r.v = (v >> 16) & 0x3fff; return r; }
 inline hEquation hEntity::equation(int i)
-    { if(i != 0) oops(); hEquation r; r.v = v | 0x40000000; return r; }
+    { hEquation r; r.v = v | 0x40000000; return r; }
 
 inline hRequest hParam::request(void)
     { hRequest r; r.v = (v >> 16); return r; }
diff --git a/src/solvespace.cpp b/src/solvespace.cpp
index a13107c..b845c89 100644
--- a/src/solvespace.cpp
+++ b/src/solvespace.cpp
@@ -426,7 +426,7 @@ bool SolveSpaceUI::OkayToStartNewFile(void) {
         case DIALOG_CANCEL:
             return false;
 
-        default: oops(); break;
+        default: ssassert(false, "Unexpected dialog choice");
     }
 }
 
@@ -557,7 +557,7 @@ void SolveSpaceUI::MenuFile(int id) {
                 ImportDxf(importFile);
             } else if(Extension(importFile) == "dwg") {
                 ImportDwg(importFile);
-            } else oops();
+            } else ssassert(false, "Unexpected extension of file to import");
 
             SS.GenerateAll(SolveSpaceUI::GENERATE_UNTIL_ACTIVE);
             SS.ScheduleShowTW();
@@ -569,7 +569,7 @@ void SolveSpaceUI::MenuFile(int id) {
             SS.Exit();
             break;
 
-        default: oops();
+        default: ssassert(false, "Unexpected menu ID");
     }
 
     SS.UpdateWindowTitle();
@@ -796,7 +796,7 @@ void SolveSpaceUI::MenuAnalyze(int id) {
             break;
         }
 
-        default: oops();
+        default: ssassert(false, "Unexpected menu ID");
     }
 }
 
@@ -823,7 +823,7 @@ void SolveSpaceUI::MenuHelp(int id) {
 );
             break;
 
-        default: oops();
+        default: ssassert(false, "Unexpected menu ID");
     }
 }
 
diff --git a/src/solvespace.h b/src/solvespace.h
index 40fe7cf..cd77821 100644
--- a/src/solvespace.h
+++ b/src/solvespace.h
@@ -74,8 +74,6 @@ struct FT_FaceRec_;
     } while(0)
 #endif
 
-#define oops() ssassert(false, "oops() called")
-
 #ifndef isnan
 #   define isnan(x) (((x) != (x)) || (x > 1e11) || (x < -1e11))
 #endif
diff --git a/src/srf/boolean.cpp b/src/srf/boolean.cpp
index d064952..54aee88 100644
--- a/src/srf/boolean.cpp
+++ b/src/srf/boolean.cpp
@@ -43,7 +43,7 @@ SCurve SCurve::MakeCopySplitAgainst(SShell *agnstA, SShell *agnstB,
     ret.pts = {};
 
     SCurvePt *p = pts.First();
-    if(!p) oops();
+    ssassert(p != NULL, "Cannot split an empty curve");
     SCurvePt prev = *p;
     ret.pts.Add(p);
     p = pts.NextAfter(p);
@@ -221,7 +221,7 @@ static bool KeepRegion(int type, bool opA, int shell, int orig)
                 return (inShell && !inFace) || inSame;
             }
 
-        default: oops();
+        default: ssassert(false, "Unexpected shell type");
     }
 }
 static bool KeepEdge(int type, bool opA,
@@ -295,7 +295,7 @@ static void DEBUGEDGELIST(SEdgeList *sel, SSurface *surf) {
 void SSurface::FindChainAvoiding(SEdgeList *src, SEdgeList *dest,
                                  SPointList *avoid)
 {
-    if(src->l.n < 1) oops();
+    ssassert(src->l.n > 0, "Need at least one edge");
     // Start with an arbitrary edge.
     dest->l.Add(&(src->l.elem[0]));
     src->l.ClearTags();
diff --git a/src/srf/curve.cpp b/src/srf/curve.cpp
index ba54afc..4adede3 100644
--- a/src/srf/curve.cpp
+++ b/src/srf/curve.cpp
@@ -764,7 +764,7 @@ SSurface *SCurve::GetSurfaceA(SShell *a, SShell *b) {
         return b->surface.FindById(surfA);
     } else if(source == FROM_INTERSECTION) {
         return a->surface.FindById(surfA);
-    } else oops();
+    } else ssassert(false, "Unexpected curve source");
 }
 
 SSurface *SCurve::GetSurfaceB(SShell *a, SShell *b) {
@@ -774,7 +774,7 @@ SSurface *SCurve::GetSurfaceB(SShell *a, SShell *b) {
         return b->surface.FindById(surfB);
     } else if(source == FROM_INTERSECTION) {
         return b->surface.FindById(surfB);
-    } else oops();
+    } else ssassert(false, "Unexpected curve source");
 }
 
 //-----------------------------------------------------------------------------
diff --git a/src/srf/ratpoly.cpp b/src/srf/ratpoly.cpp
index 2f6a0bb..bfbb951 100644
--- a/src/srf/ratpoly.cpp
+++ b/src/srf/ratpoly.cpp
@@ -51,7 +51,7 @@ double SolveSpace::Bernstein(int k, int deg, double t)
             }
             break;
     }
-    oops();
+    ssassert(false, "Unexpected degree of spline");
 }
 
 double SolveSpace::BernsteinDerivative(int k, int deg, double t)
@@ -90,7 +90,7 @@ double SolveSpace::BernsteinDerivative(int k, int deg, double t)
             }
             break;
     }
-    oops();
+    ssassert(false, "Unexpected degree of spline");
 }
 
 Vector SBezier::PointAt(double t) {
@@ -222,7 +222,7 @@ void SBezier::SplitAt(double t, SBezier *bef, SBezier *aft) {
             *aft = SBezier::From(cts, ct12_23, ct23, ct[3]);
             break;
         }
-        default: oops();
+        default: ssassert(false, "Unexpected degree of spline");
     }
 }
 
@@ -308,7 +308,7 @@ void SBezier::MakePwlInitialWorker(List<Vector> *l, double ta, double tb, double
     double d = max({
                    pm1.DistanceToLine(pa, dir),
                    pm2.DistanceToLine(pa, dir),
-                   pm3.DistanceToLine(pa, dir) 
+                   pm3.DistanceToLine(pa, dir)
                 });
 
     double step = 1.0/SS.GetMaxSegments();
diff --git a/src/srf/raycast.cpp b/src/srf/raycast.cpp
index 808287f..7dbca01 100644
--- a/src/srf/raycast.cpp
+++ b/src/srf/raycast.cpp
@@ -172,7 +172,7 @@ void SSurface::SplitInHalf(bool byU, SSurface *sa, SSurface *sb) {
             break;
         }
 
-        default: oops();
+        default: ssassert(false, "Unexpected degree of spline");
     }
 
     sa->UnWeightControlPoints();
diff --git a/src/srf/surfinter.cpp b/src/srf/surfinter.cpp
index 79d71cd..998ce4c 100644
--- a/src/srf/surfinter.cpp
+++ b/src/srf/surfinter.cpp
@@ -97,8 +97,8 @@ void SSurface::AddExactIntersectionCurve(SBezier *sb, SSurface *srfB,
         }
     }
 #endif // 0
-    // Nothing should be generating zero-len edges.
-    if((sb->Start()).Equals(sb->Finish())) oops();
+    ssassert(!(sb->Start()).Equals(sb->Finish()),
+             "Unexpected zero-length edge");
 
     split.source = SCurve::FROM_INTERSECTION;
     into->curve.AddAndAssignId(&split);
diff --git a/src/style.cpp b/src/style.cpp
index 91caaea..20008ec 100644
--- a/src/style.cpp
+++ b/src/style.cpp
@@ -554,7 +554,7 @@ void TextWindow::ScreenChangeStyleMetric(int link, uint32_t v) {
             meaning = EDIT_STYLE_WIDTH;
             break;
 
-        default: oops();
+        default: ssassert(false, "Unexpected link");
     }
 
     std::string edit_value;
@@ -588,9 +588,7 @@ void TextWindow::ScreenChangeStyleColor(int link, uint32_t v) {
     } else if(link == 'f') {
         em = EDIT_STYLE_FILL_COLOR;
         rgb = s->fillColor;
-    } else {
-        oops();
-    }
+    } else ssassert(false, "Unexpected link");
     SS.TW.ShowEditControlWithColorPicker(13, rgb);
     SS.TW.edit.style = hs;
     SS.TW.edit.meaning = em;
@@ -840,7 +838,7 @@ void TextWindow::ShowStyleInfo(void) {
                 case '_': patterns[i] += "\xEE\x80\x85"; break;
                 case '-': patterns[i] += "\xEE\x80\x86"; break;
                 case '~': patterns[i] += "\xEE\x80\x87"; break;
-                default: oops();
+                default: ssassert(false, "Unexpected stipple pattern element");
             }
         } while(*(++str));
     }
diff --git a/src/textscreens.cpp b/src/textscreens.cpp
index e00b2bd..0f37fa8 100644
--- a/src/textscreens.cpp
+++ b/src/textscreens.cpp
@@ -788,10 +788,7 @@ void TextWindow::EditControlDone(const char *s) {
             if(EditControlDoneForConfiguration(s))  cnt++;
             if(EditControlDoneForPaste(s))          cnt++;
             if(EditControlDoneForView(s))           cnt++;
-            if(cnt > 1) {
-                // The identifiers were somehow assigned not uniquely?
-                oops();
-            }
+            ssassert(cnt == 1, "Expected exactly one parameter to be edited");
             break;
         }
     }
diff --git a/src/textwin.cpp b/src/textwin.cpp
index 0e9483a..9f26594 100644
--- a/src/textwin.cpp
+++ b/src/textwin.cpp
@@ -48,7 +48,7 @@ void TextWindow::MakeColorTable(const Color *in, float *out) {
     int i;
     for(i = 0; in[i].c != 0; i++) {
         int c = in[i].c;
-        if(c < 0 || c > 255) oops();
+        ssassert(c >= 0 && c <= 255, "Unexpected color index");
         out[c*3 + 0] = in[i].color.redF();
         out[c*3 + 1] = in[i].color.greenF();
         out[c*3 + 2] = in[i].color.blueF();
diff --git a/src/ttf.cpp b/src/ttf.cpp
index da9d227..c1555fc 100644
--- a/src/ttf.cpp
+++ b/src/ttf.cpp
@@ -218,7 +218,7 @@ static const FT_Outline_Funcs outline_funcs = {
 void TtfFont::PlotString(const std::string &str,
                          SBezierList *sbl, Vector origin, Vector u, Vector v)
 {
-    if(fontFace == NULL) oops();
+    ssassert(fontFace != NULL, "Expected font face to be loaded");
 
     FT_Pos dx = 0;
     for(char32_t chr : ReadUTF8(str)) {
diff --git a/src/undoredo.cpp b/src/undoredo.cpp
index 04b96ee..d21ca93 100644
--- a/src/undoredo.cpp
+++ b/src/undoredo.cpp
@@ -100,7 +100,7 @@ void SolveSpaceUI::PushFromCurrentOnto(UndoStack *uk) {
 void SolveSpaceUI::PopOntoCurrentFrom(UndoStack *uk) {
     int i;
 
-    if(uk->cnt <= 0) oops();
+    ssassert(uk->cnt > 0, "Cannot pop from empty undo stack");
     (uk->cnt)--;
     uk->write = WRAP(uk->write - 1, MAX_UNDO);
 
diff --git a/src/util.cpp b/src/util.cpp
index 7a4b942..6555570 100644
--- a/src/util.cpp
+++ b/src/util.cpp
@@ -12,7 +12,7 @@ std::string SolveSpace::ssprintf(const char *fmt, ...)
 
     va_start(va, fmt);
     int size = vsnprintf(NULL, 0, fmt, va);
-    if(size < 0) oops();
+    ssassert(size >= 0, "vsnprintf could not encode string");
     va_end(va);
 
     std::string result;
@@ -418,7 +418,7 @@ double Vector::Element(int i) {
         case 0: return x;
         case 1: return y;
         case 2: return z;
-        default: oops();
+        default: ssassert(false, "Unexpected vector element index");
     }
 }
 
@@ -514,7 +514,7 @@ Vector Vector::Normal(int which) {
         // That's the vector we return.
     } else if(which == 1) {
         n = this->Cross(n);
-    } else oops();
+    } else ssassert(false, "Unexpected vector normal index");
 
     n = n.WithMagnitude(1);