sympy/sympy — pull request #1615 by mrocklin (2012-10-27 01:29:03+00:00)
Title: Rules: Traversals stop on non-Basics

Traversals no longer fail when they encounter a non-basic in the tree

This allows ints, strings, and other standard python objects to be leaves in the expression tree. Notably this means that Basics can contain strings without fouling up the rules system.