diff --git a/remix/data0.rkt b/remix/data0.rkt
index 4413019..e281ef3 100644
--- a/remix/data0.rkt
+++ b/remix/data0.rkt
@@ -8,8 +8,11 @@
           (prefix-in remix: remix/stx0)
           remix/stx/singleton-struct0
           (for-syntax racket/base
+                      racket/syntax
                       syntax/parse
                       (prefix-in remix: remix/stx0)))
+         racket/unsafe/ops
+         racket/performance-hint
          (prefix-in remix: remix/stx0))
 
 (begin-for-syntax
@@ -34,7 +37,11 @@
              (~optional
               (~seq #:is rhs-dt:id)
               #:defaults ([rhs-dt #'#f])))
-            ...)
+            ...
+            (~optional
+             (~seq #:extensions
+                   extension ...)
+             #:defaults ([[extension 1] '()])))
        (with-syntax ([int-name (syntax-local-name)]
                      [(def-rhs ...) (generate-temporaries #'(rhs ...))])
          (syntax/loc stx
@@ -75,9 +82,8 @@
                                (if xd #'xb #'(make-rename-transformer #'xb))])
                  (quasisyntax/loc stx
                    (remix:def (remix:#%brackets x-def #,x-stx) x-def-v))))
-             ;; XXX add the ability to add other properties,
-             ;; interfaces, etc.
              (singleton-struct
+              ;; XXX some way to overload this with #:extensions
               #:property prop:procedure
               (λ (_ stx)
                 (raise-syntax-error 'int-name "Illegal in expression context" stx))
@@ -120,24 +126,80 @@
                           (remix:def (remix:#%brackets remix:stx i)
                                      (phase1:static-interface
                                       (remix:#%brackets lhs def-rhs)
-                                      ...)))))]))]))))])))
+                                      ...)))))]))]
+              extension ...))))])))
 
-(define-syntax phase0:static-interface
-  (singleton-struct
-   #:property prop:procedure
-   (λ (_ stx)
-     (raise-syntax-error 'static-interface "Illegal outside def" stx))
-   #:methods remix:gen:def-transformer
-   [(define (def-transform _ stx)
-      (syntax-parse stx
-        #:literals (remix:#%brackets)
-        [(def (remix:#%brackets me:id i:id) . body:expr)
-         (syntax/loc stx
-           (remix:def (remix:#%brackets remix:stx i)
-                      (phase1:static-interface . body)))]))]))
+(define-syntax (define-phase0-def->phase1-macro stx)
+  (syntax-parse stx
+    [(_ base:id)
+     (with-syntax ([phase0:base (format-id #'base "phase0:~a" #'base)]
+                   [phase1:base (format-id #'base "phase1:~a" #'base)])
+       (syntax/loc stx
+         (define-syntax phase0:base
+           (singleton-struct
+            #:property prop:procedure
+            (λ (_ stx)
+              (raise-syntax-error 'base "Illegal outside def" stx))
+            #:methods remix:gen:def-transformer
+            [(define (def-transform _ stx)
+               (syntax-parse stx
+                 #:literals (remix:#%brackets)
+                 [(def (remix:#%brackets me:id i:id) . body:expr)
+                  (syntax/loc stx
+                    (remix:def (remix:#%brackets remix:stx i)
+                               (phase1:base . body)))]))]))))]))
 
+(define-phase0-def->phase1-macro static-interface)
 
 (provide (rename-out [phase0:static-interface static-interface])
          (for-syntax (rename-out [phase1:static-interface static-interface])
                      gen:static-interface
-                     static-interface?))
+                     static-interface?
+                     static-interface-members))
+
+(begin-for-syntax
+  ;; XXX fill this in
+  (define-generics layout))
+
+(define-syntax phase0:layout
+  (singleton-struct
+   #:property prop:procedure
+   (λ (_ stx)
+     (raise-syntax-error 'layout "Illegal outside def" stx))
+   #:methods remix:gen:def-transformer
+   [(define (def-transform _ stx)
+      (syntax-parse stx
+        #:literals (remix:#%brackets)
+        [(def (remix:#%brackets me:id name:id)
+           f:id ...)
+         (with-syntax* ([name-alloc (format-id #f "~a-alloc" #'name)]
+                        [name-set (format-id #f "~a-set" #'name)]
+                        [(f-idx ...)
+                         (for/list ([i (in-naturals)]
+                                    [f (in-list (syntax->list #'(f ...)))])
+                           i)]
+                        [(name-f ...)
+                         (generate-temporaries #'(f ...))])
+           (syntax/loc stx
+             (begin
+               (define-syntax (name-alloc stx)
+                 (raise-syntax-error 'name-alloc "XXX alloc"))
+               (define-syntax (name-set stx)
+                 (raise-syntax-error 'name-set "XXX set"))
+               (begin-encourage-inline
+                 (define (name-f v) (unsafe-vector*-ref v f-idx))
+                 ...)
+               (define-syntax name
+                 (phase1:static-interface
+                  (remix:#%brackets #:alloc name-alloc)
+                  (remix:#%brackets #:set name-set)
+                  (remix:#%brackets #:= name-set)
+                  (remix:#%brackets f name-f)
+                  ...
+                  #:extensions
+                  #:methods gen:layout
+                  [])))))]))]))
+
+(provide (rename-out [phase0:layout layout])
+         (for-syntax gen:layout
+                     layout?))
diff --git a/remix/tests/rocket.rkt b/remix/tests/rocket.rkt
index 2e1c404..41d4014 100644
--- a/remix/tests/rocket.rkt
+++ b/remix/tests/rocket.rkt
@@ -15,7 +15,7 @@
   (#rocket.alloc [h h] [dh dh]))
  #:implements world/anim^
  (def (tick)
-  (r.= [h {r.h + r.dh}]))
+   (r.= [h {r.h + r.dh}]))
  (def (draw)
   (circle 'yellow 5)))
 
diff --git a/remix/tests/simple.rkt b/remix/tests/simple.rkt
index fe0a1c4..27c39e9 100644
--- a/remix/tests/simple.rkt
+++ b/remix/tests/simple.rkt
@@ -298,6 +298,9 @@
   {(example3^.fg.g 2) ≡ 2}
   {example3^.h ≡ 19})
 
+;; XXX show an example where it isn't an interface but any def
+;; transformer.
+
 ;; The syntax of interface members is not limited to identifiers. In
 ;; particular, #:keywords are useful. Furthermore, static-interface is
 ;; a def transformer itself, to clean up the syntax a little bit. I
@@ -311,3 +314,85 @@
   {example4^.#:key ≡ '#:key}
   {example4^.key ≡ 'key})
 
+;; A layout is a container with no sealing or representation
+;; guarantees. This means you can't necessarily protect the contents
+;; nor can you necessarily tell that you have one when you do.
+
+;; layout is a def-transformer (XXX I wish I could make it phase1
+;; macro also but it needs to define some functions that could be
+;; called)
+
+;; The most basic syntax is a list of fields, which are identifiers.
+(def [layout posn]
+  x y)
+(module+ test
+  ;; You will get an allocation function named #:alloc
+  (def p1 (posn.#:alloc [x 5] [y 7]))
+  ;; And accessors
+  {p1.x ≡ 5}
+  {p1.y ≡ 7}
+  ;; You will also get a copying function (XXX: Should it be named
+  ;; `copy`? `update`? My analogy here is with hash-set)
+  (def p2 (p1.#:set [x 8] [y {p1.y + 2}]))
+  ;; Notice that these built-in functions are keywords, so that they
+  ;; can't conflict with the fields you've defined.
+  {p2.x ≡ 8}
+  {p2.y ≡ 9}
+  ;; This is aliased to =, which I expect is nicer to use.
+  (def p3 (p1.#:= [x 8] [y {p1.y + 2}]))
+  {p3.x ≡ 8}
+  {p3.y ≡ 9})
+
+;; A layout can have a parent, which provides the guarantee that the
+;; parent's functions will work on the child. A layout has one or zero
+;; parents.
+#;
+(def [layout quat]
+  #:parent posn
+  z)
+
+;; A layout's fields may be specified as other layouts. When the first
+;; field is a layout, this is not necessarily the same thing as a
+;; parent (like C structs) but it may be.
+#;
+(def [layout circle]
+  [posn cx] r)
+
+;; A layout's fields can _actually_ just be any def transformer, and
+;; thus could be static interfaces
+#;
+(def [layout weird]
+  [example1^ e])
+
+;; Now, the big reveal, layout has an extensible representation
+;; planner system. At the moment, the only representations are
+;;
+;; layout-immutable : The default, backed by immutable vectors 
+;; layout-mutable   : Backed by mutable vectors, with mutation support
+;;
+;; I expect to produce a few more
+;;
+;; (XXX) layout-c        : Compatible with C
+;; (XXX) layout-optimize : Optimize for removing padding and have
+;;                         cache-line-aligned accesses
+;;
+;; It would be possible to make layout-c right now, but define-cstruct
+;; is really slow. It is trivial to have layout-optimize if you have
+;; layout-c, but it would not be useful to use. mflatt and I talked
+;; about a fast way of implementing them in Racket. The basic idea is
+;; to have a new type of object in the VM where the pointer goes to
+;; the middle of the allocated space which looks like
+;;
+;; [ <raw-values> | <tag> <vector layout> ]
+;;
+;; There may be necessary padding, but then the existing vector
+;; functions would work. The raw values would use computed offsets to
+;; get the values. The goal would be that parent structs would just
+;; work and it would be easy to pass to C by sorting the _racket
+;; pointers to the end.
+;;
+;; Anyways, here's a mutable example.
+#;
+(def [layout world]
+  #:rep layout-mutable
+  [circle c1] [circle c2])