(** This file is an adaptation to the [FMap] framework of a work by
  Xavier Leroy and Sandrine Blazy (used for building certified compilers).
  Keys are of type [positive], and maps are binary trees: the sequence
  of binary digits of a positive number corresponds to a path in such a tree.
  This is quite similar to the [IntMap] library, except that no path
  compression is implemented, and that the current file is simple enough to be
  self-contained. *)

(** First, some stuff about [positive] *)

Fixpoint append (i j : positive) : positive :=
    match i with
      | xH => j
      | xI ii => xI (append ii j)
      | xO ii => xO (append ii j)
    end.

Lemma append_neutral_l : forall (i : positive), append xH i = i.