Comment #16 on issue 2081 by asmeurer: Pow substitution introduces wrong answers
http://code.google.com/p/sympy/issues/detail?id=2081
But we need three types of substitution. Exact substitution (issue 2026), algebraic substitution (what it does now), and a mix that does the algebraic substitution only if the resulting power is an integer multiple of the original (issue 790, particularly comment 5).
In my opinion, the full algebraic substitution should be the default, and the others should be accessible via keyword arguments. Or at least make it that way for the ones where it is valid assumptions wise. In the other cases, perhaps the integer powers one should be default, but overridable via some algebraic=True flag.
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