sympy/sympy — pull request #2774 by zanzibar7 (2014-01-15 06:37:05+00:00)
Title: Fix a bug where subs() forgets noncommutativity

I was doing some work with rewrite systems using noncommuting variables and discovered a bug in subs().
This is a simple fix and a corresponding test. The problem was that the dummy variables in simultaneous substitutes did not preseve the parent's properties.