Comment #8 on issue 2641 by asmeurer: limit() heuristics are wrong
http://code.google.com/p/sympy/issues/detail?id=2641
For example, you can't factor out this expression into a non-Add:
In [25]: limit(log(x)*z - log(2*x)*y, x, 0)
Out[25]: zoo⋅y + zoo⋅z
In [26]: gruntz(log(x)*z - log(2*x)*y, x, 0)
Out[26]: ∞⋅sign(y - z)
So I think that instead of relying on nan popping out, which is very
fragile, that limit should not split itself across an Add (except maybe in
trivial cases, e.g., it's OK to pull out terms from an Add that are
independent of the limit variable). And as far as I know about the limit
algorithm, it should be OK to pull out multiplicative constants.
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