sympy/sympy — pull request #8028 by smichr (2014-09-12 23:13:03+00:00)
Title: sign(sqrt(1-sqrt(2))) -> I instead of being unevaluated

Numerical expression that are not not Numbers and symbolic expressions are not autoexpanded like sqrt(-2)->I*sqrt(2). So sign is made aware of this condition so sign of well defined negative args of sqrt can be returned as `I`.