sympy/sympy — pull request #1287 by goodok (2012-05-10 17:21:32+00:00)
Title: Fix mathml printer.

If anyone uses the mathml printer, the behavior of toprettyxml
was changed globally. This still could cause subtle bugs for people
who use that function.

This commit restore standard xml library after each MathML printer
as it was before.

See also discussion in previous [pull 1184](https://github.com/sympy/sympy/pull/1184#issuecomment-5532877)