'Perform the actual proof.  Store the result to prevent unnecessary
re-proving.'

def prove(self, verbose=False):

    if (self._result is None):
        (self._result, clauses) = self._prover._prove(self.goal(), self.assumptions(), verbose)
        self._clauses = clauses
        self._proof = ResolutionProverCommand._decorate_clauses(clauses)
    return self._result
