Fixpoint ndN (m:fmap):nat:=
match m with
V => 0%nat
| I m0 x _ _ =>
if exd_dec m0 x then ndN m0
else (1 + ndN m0)%nat
| L m0 _ _ _ => ndN m0
end.

(* Specifications: *)
Lemma nd_card:forall(m:fmap),
ndN m = card (fmap_to_set m).