The main data structure is a full binary tree.
A binary tree is full if each of its node has either two or zero children.
If a node has two children it is an internal node, otherwise a leaf.
Fundamental theorem: Let I be the number of internal nodes and L the number of leaves, then: L = I + 1
(It can be proved by induction on the number of internal nodes.)
This means that when we add a leaf to the tree (when a window is created), we must also add one internal node.