Converts a lark rule, (lhs, rhs, callback, options), to a Rule.

def _to_rule(self, lark_rule):
        """Converts a lark rule, (lhs, rhs, callback, options), to a Rule."""
        assert isinstance(lark_rule.origin, NT)
        assert all(isinstance(x, Symbol) for x in lark_rule.expansion)
        return Rule(
            lark_rule.origin, lark_rule.expansion,
            weight=lark_rule.options.priority if lark_rule.options and lark_rule.options.priority else 0,
            alias=lark_rule)