(** 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_inlist_swap :
forall (x y : Z) (l : Zlist),
inlist Z y l -> inlist Z x (zdrop y l) -> inlist Z y (zdrop x l).