sympy/sympy — pull request #1389 by melsophos (2012-06-27 08:14:57+00:00)
Title: Units

Some preliminaries work on the unit systems. It's not over but Aaron advised to do a pull request to help the review (see https://groups.google.com/forum/?fromgroups#!topic/sympy/XbWxpKS6S94).
The current main problem comes from the operations, since Mul does not know that it should combine units and quantities together first (see the last test for an example).