(* SUPPRESSION OF ALL OCCURRENCES of z: *)

Fixpoint Ds(s:set)(z:dart){struct s}:set:=
  match s with
     Vs => Vs
   | Is s0 x =>
       if eq_dart_dec x z then (Ds s0 z)
       else Is (Ds s0 z) x
  end.

Lemma exds_Ds:forall(s:set)(x z:dart), x <> z -> (exds s z <-> exds (Ds s x) z).