(** * Elements *)
Notation eqk := (PX.eqk (elt:= elt)).
Notation eqke := (PX.eqke (elt:= elt)).
Notation ltk := (PX.ltk (elt:= elt)).

Lemma elements_node :
forall (t1 t2:t elt) x e z l,
elements t1 ++ (x,e) :: elements t2 ++ l =
elements (Node t1 x e t2 z) ++ l.