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