(* "cardinal" of a (multi-)set: *)
Fixpoint card(s:set):nat:=
match s with
Vs => 0%nat
| Is s0 x =>
if exds_dec s0 x then card s0
else (1 + card s0)%nat
end.

Lemma exds_card_Ds:forall (s:set)(z:dart),
exds s z -> card (Ds s z) = (card s - 1)%nat.