sympy/sympy — pull request #1590 by smichr (2012-10-21 22:48:14+00:00)
Title: 1836 - subs sees expanded 2-arg mul

http://code.google.com/p/sympy/issues/detail?id=1836

Now `(2*a + 2*b).subs(a + b, 1) -> 2`