composOp should iterate under ImplArg

src/compiler/GF/Grammar/Macros.hs

@@ -535,6 +535,10 @@ composOp co trm =
      do ty' <- co ty
         return (ELin c ty')
         
+   ImplArg t -> 
+     do t' <- co t
+        return (ImplArg t')
+
    _ -> return trm -- covers K, Vr, Cn, Sort, EPatt
 
 getTableType :: TInfo -> Err Type