sympy/sympy — pull request #2791 by smichr (2014-01-18 05:55:15+00:00)
Title: do not use None in factor_list

The None tracked a term that should not be extracted
if the exponent assumptions didn't allow. But this has
to be handled differently since the "k" value is needed
if the assumptions allow. A test that caused the gh failure
and that gave the wrong answer when the solution was formulated
as just dropping the k term when k was None was added.

fixes issue 2787