Get weight for rule.

        If rule is not in :attr:`R`, returns ``None``.

def get_weight(self, rule):
        """Get weight for rule.

        If rule is not in :attr:`R`, returns ``None``.
        """
        if not issubclass(rule.__class__, (Rule, RuleLeaf)):
            raise TypeError("Rule to get weight ({}) is not subclass "
                            "of {} or {}.".format(rule, Rule, RuleLeaf))
        try:
            ind = self._R.index(rule)
            return self._W[ind]
        except:
            return None