diff --git a/doc/abi.org b/doc/abi.org
index 439ecb5..d6b6db2 100644
--- a/doc/abi.org
+++ b/doc/abi.org
@@ -132,3 +132,85 @@ evaluation of ~(fac 0)~:
 (push! 1)                  ; [] []
 (call! ktail)              ; [1] []
 #+end_src
+
+#+begin_src scheme
+(push! 3)                  ; [] []
+(push-cont! halt)          ; [3] []
+(call! fac)                ; [3] [halt]
+
+(define n (pop!))          ; [3] [halt]
+(push! n)                  ; [] [halt]
+(push-cont! fac-k0)        ; [3] [halt]
+(push! n)                  ; [3] [halt fac-k0]
+(call! zero?)              ; [3 3] [halt fac-k0]
+#<internals of zero?>      ; [3 3] [halt fac-k0]
+(define x0 (pop!))         ; [3 #f] [halt]
+(define n (pop!))          ; [3] [halt]
+(define x1 (- n 1))        ; [] [halt]
+(push! n)                  ; [] [halt]
+(push! x1)                 ; [3] [halt]
+(push-cont! fac-k1)        ; [3 2] [halt]
+(call! fac)                ; [3 2] [halt fac-k1]
+
+(define n (pop!))          ; [3 2] [halt fac-k1]
+(push! n)                  ; [3 ] [halt fac-k1]
+(push-cont! fac-k0)        ; [3 2] [halt fac-k1]
+(push! n)                  ; [3 2] [halt fac-k1 fac-k0]
+(call! zero?)              ; [3 2 2] [halt fac-k1 fac-k0]
+#<internals of zero?>      ; [3 2 2] [halt fac-k1 fac-k0]
+(define x0 (pop!))         ; [3 2 #f] [halt fac-k1]
+(define n (pop!))          ; [3 2] [halt fac-k1]
+(define x1 (- n 1))        ; [3] [halt fac-k1]
+(push! n)                  ; [3] [halt fac-k1]
+(push! x1)                 ; [3 2] [halt fac-k1]
+(push-cont! fac-k1)        ; [3 2 1] [halt fac-k1]
+(call! fac)                ; [3 2 1] [halt fac-k1 fac-k1]
+
+(define n (pop!))          ; [3 2 1] [halt fac-k1 fac-k1]
+(push! n)                  ; [3 2] [halt fac-k1 fac-k1]
+(push-cont! fac-k0)        ; [3 2 1] [halt fac-k1 fac-k1]
+(push! n)                  ; [3 2 1] [halt fac-k1 fac-k1 fac-k0]
+(call! zero?)              ; [3 2 1 1] [halt fac-k1 fac-k1 fac-k0]
+#<internals of zero?>      ; [3 2 1 1] [halt fac-k1 fac-k1 fac-k0]
+(define x0 (pop!))         ; [3 2 1 #f] [halt fac-k1 fac-k1]
+(define n (pop!))          ; [3 2 1] [halt fac-k1 fac-k1]
+(define x1 (- n 1))        ; [3 2] [halt fac-k1 fac-k1]
+(push! n)                  ; [3 2] [halt fac-k1]
+(push! x1)                 ; [3 2 1] [halt fac-k1 fac-k1]
+(push-cont! fac-k1)        ; [3 2 1 0] [halt fac-k1 fac-k1]
+(call! fac)                ; [3 2 1 0] [halt fac-k1 fac-k1 fac-k1]
+
+(define n (pop!))          ; [3 2 1 0] [halt fac-k1 fac-k1 fac-k1]
+(push! n)                  ; [3 2 1] [halt fac-k1 fac-k1 fac-k1]
+(push-cont! fac-k0)        ; [3 2 1 0] [halt fac-k1 fac-k1 fac-k1]
+(push! n)                  ; [3 2 1 0] [halt fac-k1 fac-k1 fac-k1 fac-k0]
+(call! zero?)              ; [3 2 1 0 0] [halt fac-k1 fac-k1 fac-k1 fac-k0]
+#<internals of zero?>      ; [3 2 1 0 0] [halt fac-k1 fac-k1 fac-k1 fac-k0]
+(define x0 (pop!))         ; [3 2 1 0 #t] [halt fac-k1 fac-k1 fac-k1]
+(define n (pop!))          ; [3 2 1 0] [halt fac-k1 fac-k1 fac-k1]
+(define ktail (pop-cont!)) ; [3 2 1] [halt fac-k1 fac-k1 fac-k1]
+(push! 1)                  ; [3 2 1 1] [halt fac-k1 fac-k1]
+(call! ktail)              ; [3 2 1 1] [halt fac-k1 fac-k1]
+
+(define n (pop!))          ; [3 2 1 1] [halt fac-k1 fac-k1]
+(define x2 (pop!))         ; [3 2 1] [halt fac-k1 fac-k1]
+(define x3 (* n x2))       ; [3 2] [halt fac-k1 fac-k1]
+(define ktail (pop-cont!)) ; [3 2] [halt fac-k1 fac-k1]
+(push! x3)                 ; [3 2] [halt fac-k1]
+(call! ktail)              ; [3 2 1] [halt fac-k1]
+
+(define n (pop!))          ; [3 2 1] [halt fac-k1]
+(define x2 (pop!))         ; [3 2] [halt fac-k1]
+(define x3 (* n x2))       ; [3] [halt fac-k1]
+(define ktail (pop-cont!)) ; [3] [halt fac-k1]
+(push! x3)                 ; [3] [halt]
+(call! ktail)              ; [3 2] [halt]
+
+(define n (pop!))          ; [3 2] [halt]
+(define x2 (pop!))         ; [3] [halt]
+(define x3 (* n x2))       ; [] [halt]
+(define ktail (pop-cont!)) ; [] [halt]
+(push! x3)                 ; [] []
+(call! ktail)              ; [6] []
+;; => (halt 6)
+#+end_src
diff --git a/src/Gyehoek/Stack/Syntax.hs b/src/Gyehoek/Stack/Syntax.hs
index 0423aaa..c74207a 100644
--- a/src/Gyehoek/Stack/Syntax.hs
+++ b/src/Gyehoek/Stack/Syntax.hs
@@ -25,7 +25,8 @@ import Effectful
 
 
 newtype Program = MkProgram
-  { blocks :: HashMap Name Block }
+  { blocks :: HashMap Name Block
+  }
   deriving stock (Show, Generic, Data)
 
 newtype Block = MkBlock { code :: List Instr }