Fix up stacks for type checking.

Sometimes (e.g. 'clear') the () at the end of the stack was tripping up
the unify() function.

joy/utils/polytypes.py

@@ -355,6 +355,8 @@ def type_check(name, stack):
     except KeyError:
         return # None, indicating unknown
 
+    stack = _foo_stack(stack)
+
     for fi, fo in infer(func):
         try:
           U = unify(fi, stack)
@@ -369,6 +371,16 @@ def type_check(name, stack):
     return False
 
 
+def _foo_stack(stack, s=None):
+    if s is None:
+        s = (StackJoyType(n) for n in xrange(23, 100))
+    if stack == ():
+        return next(s)
+    if isinstance(stack, tuple):
+        return tuple(_foo_stack(inner, s) for inner in stack)
+    return stack
+
+
 a0, a1, a2, a3, a4, a5, a6, a7, a8, a9 = A
 b0, b1, b2, b3, b4, b5, b6, b7, b8, b9 = B
 n0, n1, n2, n3, n4, n5, n6, n7, n8, n9 = N