Comment #1 on issue 1982 by asmeurer: use is None instead of == None http://code.google.com/p/sympy/issues/detail?id=1982
As long as we are talking about this, I was talking with someone on IRC a while back about bugs he was having in pypy because of an "a is 1" line somewhere (cython optimizes this, so it works, but it isn't guaranteed to work in other Pythons. So I think we should always use is for None, True, False, type(a), and S.everything (because those are singletons, they should work for it), but *never* for anything else.
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