Section Header
    + name := AVL_SET[E];
Section Inherit
    + parent_set:Expanded SET[E];
    + parent_avl_tree:Expanded AVL_TREE[E];
Section Public
    - add e:E <-
    - fast_add e:E <-
    - clear_count <-
    - reference_at e:E :E <-
    - item index:INTEGER :E <-
Section Public
    - create:SELF <-
    - make <-
Invariant.
[
-? {lost_nodes != NULL};
-? {lost_nodes = common_lost_nodes.at generating_type};
];