Trying to add a guard to combinators.

So they can notice if they're given a stack that doesn't match what
they're expecting.

This seems to work, but I realized that type variables in the pending
expression need to be update()'d too.  hmm...

test/test_type_inference.py

@@ -54,9 +54,15 @@ class TestCombinators(TestMixin, unittest.TestCase):
     self.assertEqualTypeStructure(infr(expression), f)
 
   def test_cons_dip(self):
-    expression = a1, (cons, s0), dip  # a1 [cons] dip
-    # (a0 [...0] -- [a0 ...0] a1)
-    f = ((s0, (a0, s1)), (a1, ((a0, s0), s1)))
+    expression = (cons, s0), dip  # [cons] dip
+    # (a2 [...1] a1 -- [a2 ...1] a3)
+    f =  (a1, (s1, (a2, s2))), (a1, ((a2, s1), s2))
+    # shouldn't a3 be a1?
+    self.assertEqualTypeStructure(infr(expression), [f])
+
+  def test_dip(self):
+    expression = dip,
+    f = ((s1, (a1, s2)), (a2, s2))  # (a1 [...1] -- a2)
     self.assertEqualTypeStructure(infr(expression), [f])
 
   def test_cons_dipd(self):
@@ -163,4 +169,4 @@ class TestYin(TestMixin, unittest.TestCase):
 
 
 if __name__ == '__main__':
-    unittest.main()
+    unittest.main(defaultTest='TestCombinators.test_cons_dip')