@Override
    public boolean equals(Object o) {
        boolean isNotTarget = ! (o instanceof TinyGameNode);
        if ( isNotTarget ) {
            throw new RuntimeException("比較不能なオブジェクトが渡されました: " + o);
        }

        TinyGameNode node = (TinyGameNode) o;
        boolean isMatched = left == node.left && right == node.right;

        return isMatched;
    }