Now works, just had to be lazy in the lookup

3or4/Interpreter/Self.hs

@@ -69,7 +69,11 @@ lookupExp =
   "\\c. rec lookup = \\xs. case xs of \
   \ { Cons(b,bs) -> \
   \     case b of \
-  \       { Branch(c', xs, e') -> if (equal c c') Pair(xs,e') (lookup bs) } \
+  \       { Branch(c', xs, e') -> case (equal c c') of \
+  \         { True() -> Pair(xs,e') \
+  \         ; False() -> lookup bs \
+  \         } \
+  \       } \
   \ }"
 
 substsExp :: Exp