val empty : 'a treeval is_empty : 'a tree -> boolval height : 'a tree -> intval root : 'a tree -> 'aval singleton_tree : 'a -> 'a treeval iter : ('a -> unit) -> 'a tree -> unitval fold : ('a -> 'b -> 'b) -> 'a tree -> 'b -> 'bPrivate.AvlTreeAvltree