public class Tree<T> : where T : Tree<T>
{
    T parent;
    List<T> children;

    public Tree(T parent)
    {
        this.parent = parent;
        this.children = new List<T>();
        if( parent!=null ) { parent.children.Add(this as T); }
    }
    public bool IsRoot { get { return parent == null; } }
    public bool IsLeaf { get { return children.Count == 0; } }
}