(** Multiply all elements except first occurance of x *)
Definition zmultDrop (x : Z) (l : Zlist) := zproduct (zdrop x l).

Lemma zmultdrop_cons_neq :
forall (p q : Z) (l : Zlist),
p <> q -> zmultDrop p (Cons Z q l) = (q * zmultDrop p l)%Z.