--- a/lib/ur/monad.ur	Sat Aug 22 16:32:31 2009 -0400
+++ b/lib/ur/monad.ur	Tue Aug 25 13:57:56 2009 -0400
@@ -34,6 +34,19 @@
        (fn _ _ => return i)
        [_] fl
 
+fun foldR3 [K] [m] (_ : monad m) [tf1 :: K -> Type] [tf2 :: K -> Type] [tf3 :: K -> Type] [tr :: {K} -> Type]
+           (f : nm :: Name -> t :: K -> rest :: {K}
+                -> [[nm] ~ rest] =>
+            tf1 t -> tf2 t -> tf3 t -> tr rest -> m (tr ([nm = t] ++ rest)))
+           (i : tr []) [r :: {K}] (fl : folder r) =
+    Top.fold [fn r :: {K} => $(map tf1 r) -> $(map tf2 r) -> $(map tf3 r) -> m (tr r)]
+       (fn [nm :: Name] [t :: K] [rest :: {K}] [[nm] ~ rest] 
+                        (acc : _ -> _ -> _ -> m (tr rest)) r1 r2 r3 =>
+           acc' <- acc (r1 -- nm) (r2 -- nm) (r3 -- nm);
+           f [nm] [t] [rest] ! r1.nm r2.nm r3.nm acc')
+       (fn _ _ _ => return i)
+       [_] fl
+
 fun mapR [K] [m] (_ : monad m) [tf :: K -> Type] [tr :: K -> Type]
          (f : nm :: Name -> t :: K -> tf t -> m (tr t)) =
     @@foldR [m] _ [tf] [fn r => $(map tr r)]