--- a/lib/ur/top.urs	Wed Jun 03 15:07:46 2015 +0200
+++ b/lib/ur/top.urs	Wed Jun 03 09:55:37 2015 -0400
@@ -35,7 +35,7 @@
 con thd3 = K1 ==> K2 ==> K3 ==> fn t :: (K1 * K2 * K3) => t.3
 
 (* Convert a record of n Units into a type-level record where
-   each field has the same value (which describes a uniformly
+o   each field has the same value (which describes a uniformly
    typed record) *)
 con mapU = K ==> fn f :: K => map (fn _ :: Unit => f)
 
@@ -290,3 +290,10 @@
 
 val max : t ::: Type -> ord t -> t -> t -> t
 val min : t ::: Type -> ord t -> t -> t -> t
+
+val assert : t ::: Type
+             -> bool   (* Did we avoid something bad? *)
+             -> string (* Explanation of the bad thing *)
+             -> string (* Source location of the bad thing *)
+             -> t      (* Return this value if all went well. *)
+             -> t