'process a -> b rule'

def process_rule(self, a, b):

    if ((not a) or isinstance(b, bool)):
        return
    if isinstance(a, bool):
        return
    if ((a, b) in self._rules_seen):
        return
    else:
        self._rules_seen.add((a, b))
    try:
        self._process_rule(a, b)
    except TautologyDetected:
        pass
