sympy/sympy — pull request #2135 by asmeurer (2013-05-25 01:03:16+00:00)
Title: Print unnamed Dummy symbols more prominently

Previously, they were just printed as _12 or pretty printed as just 12, which
looked like a number in an expression. This way, if a Dummy leaks out, it is
clear.

I didn't test this, and also it wasn't tested before (no tests failed).
Should it be tested?