sympy/sympy — pull request #975 by astraw (2012-01-14 21:39:11+00:00)
Title: improvements to DeferredVector

Following [some](http://groups.google.com/group/sympy/browse_thread/thread/e70aa378c02ea029) [discussion](http://groups.google.com/group/sympy/browse_thread/thread/9f9934d7b596ea4a), I updated DeferredVector a bit to address the points that I saw raised. In particular, I added a test that elicits the failure mode reported by Matthew Bret and a fix is made for it. I also added a docstring and some doctests.