Comment #7 on issue 2245 by asmeurer: assumption while canceling the polynomials
http://code.google.com/p/sympy/issues/detail?id=2245
I agree. To me, cancel() is not so much a "simplification" function in the sense that it will make the expression simpler, but it is one in the sense that it will reduce an identically 0 expression to 0 (assuming it is just rational function). This is because what cancel() really is is a canonization function, i.e., cancel(rational function) will always return the same canonical form given the same rational function written in different ways (assuming it always makes the numerator or denominator monic; I don't remember if it does).
If some series code is calling cancel() incorrectly, I think we should rather look at using some other simplification function that doesn't lose singularities in that place in the code.
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