Section Header
    + name := AVL_SET_NODE[E];
    - comment := "Auxiliary class to implement AVL_SET.";
This a classic implementation of an AVL tree (balanced tree first
designed by Adelson-Velskii and Landis, 1960)
Section Inherit
    + parent_avl_tree_node:Expanded AVL_TREE_NODE[E];
    - parent_any_avl_set_node:ANY_AVL_SET_NODE :=
Section AVL_SET
Creation:
    - create i:E :SELF <-
    - make i:E <-