What is the essence of a binary tree of a binary tree?

Question

Delphi

In general, it is very necessary that the essence of the "Binary Tree of the Binary Tree" be explained to me. What does it represent? What to take for nodes in such a tree? How to write an ordinary binary tree, I know, but a binary binary, I have no idea .. In general, I need your advice

Answer 1

Each node of such a tree stores a link to the root of another ordinary tree (each node has its own subtree).