(* s1 INCLUDED IN (OR OBS. EQUAL TO) s2 *)
Inductive incls(s1 s2:set):Prop:=
exds2 : (forall z:dart, exds s1 z -> exds s2 z)
-> incls s1 s2.
(* USEFUL: *)

Lemma set_minus_s_Ds :forall(s1 s2:set)(x:dart),
~ exds s1 x -> exds s2 x ->
set_minus s1 (Ds s2 x) = set_minus s1 s2.