diff --git a/lenses/core.rkt b/lenses/core.rkt
deleted file mode 100644
index e31bf4a..0000000
--- a/lenses/core.rkt
+++ /dev/null
@@ -1,190 +0,0 @@
-#lang racket
-
-(require racket/match fancy-app unstable/contract unstable/sequence)
-
-(provide lens/c
-         make-lens
-         let-lens
-         lens-view
-         lens-view*
-         lens-set
-         lens-set*
-         lens-transform
-         lens-transform*
-         lens-compose
-         lens-thrush
-         identity-lens
-         lens-struct
-         lens-proc
-         )
-
-(module+ test
-  (require rackunit))
-
-
-(define (lens-proc/c input subcomponent)
-  (-> input
-      (values subcomponent
-              (-> subcomponent
-                  input))))
-
-(define lens-2-val-context-key
-  (make-continuation-mark-key 'lens-2-val-context-key))
-
-(define-syntax-rule (let/immediate-mark key-expr val-id body-expr ...)
-  (call-with-immediate-continuation-mark key-expr (lambda (val-id) body-expr ...)))
-
-(struct lens-struct (proc)
-  #:property prop:procedure
-  (lambda (this target)
-    (let/immediate-mark lens-2-val-context-key lens-2-val-context?
-      (if lens-2-val-context?
-          ((lens-struct-proc this) target)
-          (lens-view (lens-struct-proc this) target)))))
-
-(define (lens-proc lns)
-  (match lns
-    [(lens-struct proc) proc]
-    [(? procedure?  proc) proc]))
-
-(define (lens/c target/c view/c)
-  (define proc/c (lens-proc/c target/c view/c))
-  (if/c lens-struct?
-        (struct/c lens-struct proc/c)
-        proc/c))
-
-(module+ test
-  (define list-lens/c (lens/c list? any/c))
-  (check-true (contract? list-lens/c))
-  (check-false (flat-contract? list-lens/c)))
-
-
-(define ((make-lens getter setter) v)
-  (values (getter v)
-          (setter v _))) ; fancy-app
-
-(define identity-lens
-  (values _ identity)) ; fancy-app
-
-(module+ test
-  (define (set-first l v)
-    (list* v (rest l)))
-  (define (set-second l v)
-    (list* (first l) v (rest (rest l))))
-  (define test-list '(1 2 3))
-  (define first-lens (make-lens first set-first))
-  (define second-lens (make-lens second set-second))
-  (check-equal? (lens-view first-lens test-list) 1)
-  (check-equal? (lens-set first-lens test-list 'a) '(a 2 3))
-  (check-equal? (lens-view identity-lens 3) 3)
-  (check-equal? (lens-set identity-lens 3 4) 4)
-  (check-equal? (lens-compose) identity-lens)
-  (define first* (lens-struct first-lens))
-  (check-equal? (first* test-list) 1)
-  (check-equal? (lens-view first* test-list) 1)
-  (check-equal? (lens-set first* test-list 'a) '(a 2 3))
-  )
-
-
-(define-syntax-rule (let-lens (view setter) lens-call-expr body ...)
-  (let-values ([(view setter) (with-continuation-mark lens-2-val-context-key #t
-                                lens-call-expr)])
-    body ...))
-
-(module+ test
-  (let-lens (view-first setter-first) (first-lens '(1 2 3 4 5))
-    (check-eqv? view-first 1)
-    (check-equal? (setter-first 'a) '(a 2 3 4 5))))
-
-
-(define (lens-view lens v)
-  (let-lens (view _) (lens v)
-    view))
-
-(define (lens-set lens v x)
-  (let-lens (_ setter) (lens v)
-    (setter x)))
-
-(define (lens-view* v . lenses)
-  (for/fold ([v v]) ([lens (in-list lenses)])
-    (lens-view lens v)))
-
-(define (lens-set* v . lenses/xs)
-  (unless (even? (length lenses/xs))
-    (error 'lens-set*
-           "expected an even number of association elements\n  association elements: ~v"
-           lenses/xs))
-  (for/fold ([v v]) ([lens/x (in-slice 2 lenses/xs)])
-    (match-define (list lens x) lens/x)
-    (lens-set lens v x)))
-
-(module+ test
-  (check-eqv? (lens-view first-lens '(1 2 3)) 1)
-  (check-equal? (lens-view* '((1 2) 3) first-lens second-lens) 2)
-  (check-equal? (lens-set first-lens '(1 2 3) 'a) '(a 2 3))
-  (check-equal? (lens-set* '(1 2 3) first-lens 10 second-lens 20) '(10 20 3))
-  )
-
-
-(define (lens-transform lens f v)
-  (let-lens (view setter) (lens v)
-    (setter (f view))))
-
-(define (lens-transform* v . lenses/fs)
-  (unless (even? (length lenses/fs))
-    (error 'lens-transform*
-           "expected an even number of association elements\n  association elements: ~v"
-           lenses/fs))
-  (for/fold ([v v]) ([lens/f (in-slice 2 lenses/fs)])
-    (match-define (list lens f) lens/f)
-    (lens-transform lens f v)))
-
-(module+ test
-  (check-equal? (lens-transform first-lens number->string '(1 2 3)) '("1" 2 3))
-  (check-equal? (lens-transform* '(1 2 3) first-lens number->string second-lens (* 10 _)) '("1" 20 3))
-  )
-
-
-(define ((lens-compose2 sub-lens super-lens) v)
-  (let-lens (super-view super-setter) (super-lens v)
-    (let-lens (sub-view sub-setter) (sub-lens super-view)
-      (values sub-view
-              (compose super-setter sub-setter)))))
-
-(module+ test
-  (define first-of-second-lens (lens-compose first-lens second-lens))
-  (define first-of-second-lens* (lens-thrush second-lens first-lens))
-  (define test-alist '((a 1) (b 2) (c 3)))
-  (check-eq? (lens-view first-of-second-lens test-alist) 'b)
-  (check-equal? (lens-set first-of-second-lens test-alist 'B)
-                '((a 1) (B 2) (c 3)))
-  (let-lens [val ctxt] (first-of-second-lens* test-alist)
-    (check-equal? val 'b)
-    (check-equal? (ctxt 'B) '((a 1) (B 2) (c 3))))
-  )
-
-
-(define ((generalize-operator op) v . vs)
-  (if (empty? vs)
-      v
-      (foldl (λ (next-v previous) (op previous next-v)) v vs)))
-
-(module+ test
-  (define (num-append2 n m)
-    (+ (* 10 n) m))
-  (define num-append (generalize-operator num-append2))
-  (check-eqv? (num-append 1 2 3 4 5) 12345)
-  (check-eqv? (num-append 1) 1))
-
-
-(define lens-compose-proc (generalize-operator lens-compose2))
-
-(define lens-compose
-  (case-lambda
-    [() identity-lens]
-    [(v . vs)
-     (apply lens-compose-proc v vs)]))
-
-(define (lens-thrush . args)
-  (apply lens-compose (reverse args)))
-
diff --git a/lenses/core/base.rkt b/lenses/core/base.rkt
new file mode 100644
index 0000000..01ab8be
--- /dev/null
+++ b/lenses/core/base.rkt
@@ -0,0 +1,54 @@
+#lang racket
+
+(require fancy-app)
+
+(module+ test
+  (require rackunit))
+
+(provide let-lens
+         make-lens
+         (struct-out lens-struct))
+
+
+(define lens-2-val-context-key
+  (make-continuation-mark-key 'lens-2-val-context-key))
+
+(define-syntax-rule (let/immediate-mark [val-id key-expr] body-expr ...)
+  (call-with-immediate-continuation-mark key-expr (lambda (val-id) body-expr ...)))
+
+
+(define (first-value v _) v)
+
+
+(struct lens-struct (proc)
+  #:property prop:procedure
+  (lambda (this target)
+    (let/immediate-mark [lens-2-val-context? lens-2-val-context-key]
+      (if lens-2-val-context?
+          ((lens-struct-proc this) target)
+          (call-with-values (thunk ((lens-struct-proc this) target))
+                            first-value)))))
+
+(define (lens-proc lns)
+  (match lns
+    [(lens-struct proc) proc]
+    [(? procedure?  proc) proc]))
+
+(define ((make-lens getter setter) v)
+  (values (getter v)
+          (setter v _))) ; fancy-app
+
+
+(define-syntax-rule (let-lens (view setter) lens-call-expr body ...)
+  (let-values ([(view setter) (with-continuation-mark lens-2-val-context-key #t
+                                lens-call-expr)])
+    body ...))
+
+(module+ test
+  (define (set-first l v)
+    (list* v (rest l)))
+  (define first-lens (make-lens first set-first))
+  (let-lens (view-first setter-first) (first-lens '(1 2 3 4 5))
+    (check-eqv? view-first 1)
+    (check-equal? (setter-first 'a) '(a 2 3 4 5))))
+
diff --git a/lenses/core/compose.rkt b/lenses/core/compose.rkt
new file mode 100644
index 0000000..b9ac665
--- /dev/null
+++ b/lenses/core/compose.rkt
@@ -0,0 +1,47 @@
+#lang racket
+
+(require fancy-app
+         "base.rkt"
+         "identity.rkt")
+
+(module+ test
+  (require rackunit
+           "view-set.rkt"))
+
+(provide lens-compose
+         lens-thrush)
+
+
+(define ((lens-compose2 sub-lens super-lens) v)
+  (let-lens (super-view super-setter) (super-lens v)
+    (let-lens (sub-view sub-setter) (sub-lens super-view)
+      (values sub-view
+              (compose super-setter sub-setter)))))
+
+
+(define lens-compose
+  (compose (foldr lens-compose2 identity-lens _) list))
+
+
+(module+ test
+  (define (set-first l v)
+    (list* v (rest l)))
+  (define first-lens (make-lens first set-first))
+  (define (set-second l v)
+    (list* (first l) v (rest (rest l))))
+  (define second-lens (make-lens second set-second))
+  (define first-of-second-lens (lens-compose first-lens second-lens))
+  (define test-alist '((a 1) (b 2) (c 3)))
+  (check-eq? (lens-view first-of-second-lens test-alist) 'b)
+  (check-equal? (lens-set first-of-second-lens test-alist 'B)
+                '((a 1) (B 2) (c 3))))
+
+
+(define (lens-thrush . args)
+  (apply lens-compose (reverse args)))
+
+(module+ test
+  (define first-of-second-lens* (lens-thrush second-lens first-lens))
+  (let-lens [val ctxt] (first-of-second-lens* test-alist)
+    (check-equal? val 'b)
+    (check-equal? (ctxt 'B) '((a 1) (B 2) (c 3)))))
diff --git a/lenses/core/contract.rkt b/lenses/core/contract.rkt
new file mode 100644
index 0000000..d1f03ff
--- /dev/null
+++ b/lenses/core/contract.rkt
@@ -0,0 +1,28 @@
+#lang racket
+
+(require unstable/contract
+         "base.rkt")
+
+(module+ test
+  (require rackunit))
+
+
+(define (lens-proc/c input subcomponent)
+  (-> input
+      (values subcomponent
+              (-> subcomponent
+                  input))))
+
+(define (lens/c target/c view/c)
+  (define proc/c (lens-proc/c target/c view/c))
+  (if/c lens-struct?
+        (struct/c lens-struct proc/c)
+        proc/c))
+
+
+(module+ test
+  (define list-lens/c (lens/c list? any/c))
+  (check-true (contract? list-lens/c))
+  (check-false (flat-contract? list-lens/c)))
+
+
diff --git a/lenses/core/identity.rkt b/lenses/core/identity.rkt
new file mode 100644
index 0000000..e679bc8
--- /dev/null
+++ b/lenses/core/identity.rkt
@@ -0,0 +1,18 @@
+#lang racket
+
+(require fancy-app)
+
+(module+ test
+  (require rackunit
+           "view-set.rkt"))
+
+(provide identity-lens)
+
+
+(define identity-lens
+  (values _ identity))
+
+
+(module+ test
+  (check-equal? (lens-view identity-lens 'foo) 'foo)
+  (check-equal? (lens-set identity-lens 'foo 'bar) 'bar))
diff --git a/lenses/core/main.rkt b/lenses/core/main.rkt
new file mode 100644
index 0000000..f76d3f2
--- /dev/null
+++ b/lenses/core/main.rkt
@@ -0,0 +1,10 @@
+#lang racket
+
+(require mischief)
+
+(require/provide "base.rkt"
+                 "view-set.rkt"
+                 "contract.rkt"
+                 "transform.rkt"
+                 "identity.rkt"
+                 "compose.rkt")
diff --git a/lenses/core/transform.rkt b/lenses/core/transform.rkt
new file mode 100644
index 0000000..6e415f6
--- /dev/null
+++ b/lenses/core/transform.rkt
@@ -0,0 +1,42 @@
+#lang racket
+
+(require unstable/sequence
+         "base.rkt")
+
+(module+ test
+  (require rackunit
+           fancy-app))
+
+(provide lens-transform
+         lens-transform*)
+
+
+(define (lens-transform lens f v)
+  (let-lens (view setter) (lens v)
+    (setter (f view))))
+
+(module+ test
+  (define (set-first l v)
+    (list* v (rest l)))
+  (define first-lens (make-lens first set-first))
+  (check-equal? (lens-transform first-lens number->string '(1 2 3))
+                '("1" 2 3)))
+  
+
+(define (lens-transform* v . lenses/fs)
+  (unless (even? (length lenses/fs))
+    (error 'lens-transform*
+           "expected an even number of association elements\n  association elements: ~v"
+           lenses/fs))
+  (for/fold ([v v]) ([lens/f (in-slice 2 lenses/fs)])
+    (match-define (list lens f) lens/f)
+    (lens-transform lens f v)))
+
+(module+ test
+  (define (set-second l v)
+    (list* (first l) v (rest (rest l))))
+  (define second-lens (make-lens second set-second))
+  (check-equal? (lens-transform* '(1 2 3)
+                                 first-lens number->string
+                                 second-lens (* 10 _))
+                '("1" 20 3)))
diff --git a/lenses/core/view-set.rkt b/lenses/core/view-set.rkt
new file mode 100644
index 0000000..73d4247
--- /dev/null
+++ b/lenses/core/view-set.rkt
@@ -0,0 +1,32 @@
+#lang racket
+
+(require unstable/sequence
+         fancy-app
+         "base.rkt")
+
+(provide lens-view
+         lens-set
+         lens-view*
+         lens-set*)
+
+
+(define (lens-view lens v)
+  (let-lens (view _) (lens v)
+    view))
+
+(define (lens-set lens v x)
+  (let-lens (_ setter) (lens v)
+    (setter x)))
+
+(define (lens-view* v . lenses)
+  (for/fold ([v v]) ([lens (in-list lenses)])
+    (lens-view lens v)))
+
+(define (lens-set* v . lenses/xs)
+  (unless (even? (length lenses/xs))
+    (error 'lens-set*
+           "expected an even number of association elements\n  association elements: ~v"
+           lenses/xs))
+  (for/fold ([v v]) ([lens/x (in-slice 2 lenses/xs)])
+    (match-define (list lens x) lens/x)
+    (lens-set lens v x)))
diff --git a/lenses/list.rkt b/lenses/list.rkt
deleted file mode 100644
index cac3817..0000000
--- a/lenses/list.rkt
+++ /dev/null
@@ -1,16 +0,0 @@
-#lang racket/base
-
-(require "list/car-cdr.rkt"
-         "list/list-ref-take-drop.rkt"
-         "list/cadr-etc.rkt"
-         "list/assoc.rkt"
-         )
-
-(provide (all-from-out
-          "list/car-cdr.rkt"
-          "list/list-ref-take-drop.rkt"
-          "list/cadr-etc.rkt"
-          "list/assoc.rkt"
-          )
-         (rename-out [list-ref-lens list-lens]))
-
diff --git a/lenses/list/assoc.rkt b/lenses/list/assoc.rkt
index 9e5b4b6..49b03eb 100644
--- a/lenses/list/assoc.rkt
+++ b/lenses/list/assoc.rkt
@@ -3,7 +3,7 @@
 (provide assoc-lens assv-lens assq-lens)
 
 (require racket/list
-         "../core.rkt"
+         "../core/main.rkt"
          )
 (module+ test
   (require rackunit))
diff --git a/lenses/list/cadr-etc.rkt b/lenses/list/cadr-etc.rkt
index f6d96e0..469c9d6 100644
--- a/lenses/list/cadr-etc.rkt
+++ b/lenses/list/cadr-etc.rkt
@@ -1,7 +1,7 @@
 #lang racket/base
 
 (require syntax/parse/define
-         "../core.rkt"
+         "../core/main.rkt"
          "car-cdr.rkt"
          (for-syntax racket/base
                      racket/syntax
diff --git a/lenses/list/car-cdr.rkt b/lenses/list/car-cdr.rkt
index e26686e..d534ad7 100644
--- a/lenses/list/car-cdr.rkt
+++ b/lenses/list/car-cdr.rkt
@@ -4,7 +4,7 @@
 
 (require racket/match
          fancy-app
-         "../core.rkt"
+         "../core/main.rkt"
          )
 
 (define (car-lens v)
diff --git a/lenses/list/list-ref-take-drop.rkt b/lenses/list/list-ref-take-drop.rkt
index b0906f4..4494cc2 100644
--- a/lenses/list/list-ref-take-drop.rkt
+++ b/lenses/list/list-ref-take-drop.rkt
@@ -18,7 +18,7 @@
 (require racket/list
          (only-in srfi/1 append-reverse)
          fancy-app
-         "../core.rkt"
+         "../core/main.rkt"
          "car-cdr.rkt"
          )
 (module+ test
diff --git a/lenses/list/main.rkt b/lenses/list/main.rkt
new file mode 100644
index 0000000..eb672b8
--- /dev/null
+++ b/lenses/list/main.rkt
@@ -0,0 +1,13 @@
+#lang racket/base
+
+(require "car-cdr.rkt"
+         "list-ref-take-drop.rkt"
+         "cadr-etc.rkt"
+         "assoc.rkt")
+
+(provide (all-from-out
+          "car-cdr.rkt"
+          "list-ref-take-drop.rkt"
+          "cadr-etc.rkt"
+          "assoc.rkt")
+         (rename-out [list-ref-lens list-lens]))
diff --git a/lenses/main.rkt b/lenses/main.rkt
index 54a8d45..4195758 100644
--- a/lenses/main.rkt
+++ b/lenses/main.rkt
@@ -3,7 +3,7 @@
 (require mischief)
 
 (require/provide
- "core.rkt"
- "list.rkt"
+ "core/main.rkt"
+ "list/main.rkt"
  "syntax.rkt"
  "syntax-keyword.rkt")
diff --git a/lenses/syntax-keyword.rkt b/lenses/syntax-keyword.rkt
index d535bf1..f400969 100644
--- a/lenses/syntax-keyword.rkt
+++ b/lenses/syntax-keyword.rkt
@@ -1,6 +1,6 @@
 #lang racket
 
-(require "core.rkt"
+(require "core/main.rkt"
          fancy-app
          syntax/parse)
 
diff --git a/lenses/syntax.rkt b/lenses/syntax.rkt
index bec3856..2c71ae8 100644
--- a/lenses/syntax.rkt
+++ b/lenses/syntax.rkt
@@ -2,7 +2,7 @@
 
 (require syntax/parse
          rackunit
-         "core.rkt"
+         "core/main.rkt"
          (for-syntax racket/syntax
                      syntax/stx
                      syntax/parse))