Comment #9 on issue 2070 by ronan.l...@gmail.com: All elements of .args should be Basic
http://code.google.com/p/sympy/issues/detail?id=2070
Counterexample: well, Piecewise! (BTW, I don't understand the meaning or purpose of ExprCondPair)
And yes, it would be useful to create Boolean objects S.True and S.False. There are many cases of if isinstance(expr, bool): ... else: ... in the logic code, and it would make it easier to use sympify as well.
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