* Add short-circuit code for in-place operations with self (such as

  s|=s, s&=s, s-=s, or s^=s).  Add related tests.

* Improve names for several variables and functions.

* Provide alternate table access functions (next, contains, add, and discard)
  that work with an entry argument instead of just a key.  This improves
  set-vs-set operations because we already have a hash value for each key
  and can avoid unnecessary calls to PyObject_Hash().  Provides a 5% to 20%
  speed-up for quick hashing elements like strings and integers.  Provides
  much more substantial improvements for slow hashing elements like tuples
  or objects defining a custom __hash__() function.

* Have difference operations resize() when 1/5 of the elements are dummies.
  Formerly, it was 1/6.  The new ratio triggers less frequently and only
  in cases that it can resize quicker and with greater benefit.  The right
  answer is probably either 1/4, 1/5, or 1/6.  Picked the middle value for
  an even trade-off between resize time and the space/time costs of dummy
  entries.

Lib/test/test_set.py

@@ -370,6 +370,18 @@ class TestSet(TestJointOps):
             else:
                 self.assert_(c not in self.s)
 
+    def test_inplace_on_self(self):
+        t = self.s.copy()
+        t |= t
+        self.assertEqual(t, self.s)
+        t &= t
+        self.assertEqual(t, self.s)
+        t -= t
+        self.assertEqual(t, self.thetype())
+        t = self.s.copy()
+        t ^= t
+        self.assertEqual(t, self.thetype())
+
     def test_weakref(self):
         s = self.thetype('gallahad')
         p = proxy(s)