(* THEN, THE RESULT: *)

Lemma incls_minus: forall(m:fmap)(x y:dart),
inv_hmap m -> exd m x -> exd m y ->
~expo m x y ->
let s:= fmap_to_set m in
let sx:= Iter_orb m x in
let sy:= Iter_orb m y in
incls sy (set_minus s sx).