public struct Set<T> where
    T : IEquatable<T>, IComparable<T>
{
    private class Node
    {
        public T root;
        public Set<T> leftTree;
        public Set<T> rightTree;
        public bool isBlack;
        public Node(T root, Set<T> leftTree, Set<T> rightTree, bool isBlack)
        {
            this.root = root;
            this.leftTree = leftTree;
            this.rightTree = rightTree;
            this.isBlack = isBlack;
        }
    }