(** Drop an element from a Zlist *)
Fixpoint zdrop (x : Z) (l : Zlist) {struct l} : Zlist :=
match l with
| Nil => Nil Z
| Cons h t => if Zeq_bool x h then t else Cons Z h (zdrop x t)
end.

Lemma zdrop_product :
forall (x : Z) (l : Zlist),
inlist Z x l -> (x * zproduct (zdrop x l))%Z = zproduct l.