--- a/lib/ur/list.ur	Fri Mar 27 11:19:15 2015 -0400
+++ b/lib/ur/list.ur	Fri Mar 27 11:26:06 2015 -0400
@@ -216,6 +216,16 @@
         fold []
     end
 
+fun mem [a] (_ : eq a) (x : a) =
+    let
+        fun mm ls =
+            case ls of
+                [] => False
+              | y :: ls => y = x || mm ls
+    in
+        mm
+    end
+
 fun find [a] f =
     let
         fun find' ls =