The current MutableSet.__iand__ implementation calls self.discard while
iterating on self. This creates strange problems while implementing
MutableSet with simple choices. For example, consider the attached file
which implements set by delegating either to a set or a list. In the
first cases, an exception is raised; in the second, the result is not
what is expected.

Python 2.6+ (r26:66714, Oct 22 2008, 09:21:39) 
[GCC 4.3.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from simpleset import WithSet, WithList
>>> s = WithSet([1,2])
>>> s &= ()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.6/_abcoll.py", line 290, in __iand__
    for value in self:
RuntimeError: Set changed size during iteration
>>> s = WithList([1,2])
>>> s &= ()
>>> list(s)
[2]

I suggest solving the problem by changing the implementation to:

def __iand__(self, c):
    self -= self - c:

or to

def __iand__(self, c):
    for item in self - c:
        self.discard(item)

Thanks for the report.  Fixed in r70969, r70970, r70973, and r70974.