--- a/lib/top.ur	Thu Sep 11 18:36:20 2008 -0400
+++ b/lib/top.ur	Thu Sep 11 19:59:31 2008 -0400
@@ -4,3 +4,13 @@
 fun compose (t1 ::: Type) (t2 ::: Type) (t3 ::: Type) (f1 : t2 -> t3) (f2 : t1 -> t2) (x : t1) = f1 (f2 x)
 
 fun txt (t ::: Type) (ctx ::: {Unit}) (use ::: {Type}) (sh : show t) (v : t) = cdata (show sh v)
+
+fun foldTR2 (tf1 :: Type -> Type) (tf2 :: Type -> Type) (tr :: {Type} -> Type)
+        (f : nm :: Name -> t :: Type -> rest :: {Type} -> [nm] ~ rest
+                -> tf1 t -> tf2 t -> tr rest -> tr ([nm = t] ++ rest))
+        (i : tr []) =
+        fold [fn r :: {Type} => $(mapTT tf1 r) -> $(mapTT tf2 r) -> tr r]
+                (fn (nm :: Name) (t :: Type) (rest :: {Type}) (acc : _ -> _ -> tr rest) =>
+                        [[nm] ~ rest] =>
+                        fn r1 r2 => f [nm] [t] [rest] r1.nm r2.nm (acc (r1 -- nm) (r2 -- nm)))
+                (fn _ _ => i)