sympy/sympy — pull request #1923 by skirpichev (2013-03-23 14:08:55+00:00)
Title: Implement reductions for the product of a complex conjugate pairs (issue 2296)

This is done for Mul.as_real_imag, e.g. im(x*x.conjugate()) -> 0

Can we do something else? E.g. alter assumptions code to recognize x*x.conjugate() as a real. This seems to be too costly, as was noted in the issue thread.