sympy/sympy — pull request #1605 by smichr (2012-10-24 19:29:01+00:00)
Title: 3455: Not(0)->True, Not(1)->False, else Not(arg)

In keeping with Or(2, x) == Or(2) and Or(1, x) == True,
Not() now evaluates 0 and 1 to True and False, respectively.