Given a constraint, _unpack_truisms() returns a set of constraints that must be True
        this constraint to be True.

def _unpack_truisms(self, c):
        """
        Given a constraint, _unpack_truisms() returns a set of constraints that must be True
        this constraint to be True.
        """

        try:
            op = getattr(self, '_unpack_truisms_'+c.op)
        except AttributeError:
            return set()
        return op(c)