sympy/sympy — pull request #1040 by smichr (2012-02-09 18:30:04+00:00)
Title: free_symbols method added to Unit and is_constant edits

units are a special object since they aren't recognized as numbers and yet they don't have any free symbols. So this pull request defines their free_symbols method so it returns set() and is_constant was edited slightly (comments and a redundant line removed).