public function equals(Rule $rule)
    {
        // specialized fast-case
        if ($rule instanceof self) {
            if ($this->literal1 !== $rule->literal1) {
                return false;
            }

            if ($this->literal2 !== $rule->literal2) {
                return false;
            }

            return true;
        }

        $literals = $rule->getLiterals();
        if (2 != count($literals)) {
            return false;
        }

        if ($this->literal1 !== $literals[0]) {
            return false;
        }

        if ($this->literal2 !== $literals[1]) {
            return false;
        }

        return true;
    }