Section Variables.
Definition F (x: nat): nat := (I__S (inl x)).
Definition G (x y z: nat): nat := I__S (inr (I__P (x, I__P (y, z)))).
Definition Fs E := map F (Vars__de E).
Definition Gs (E: list deq) :=
flat_map (fun e =>
match e with
| (x *ₑ y =ₑ z) => [G x y z]
| _ => nil
end) E.

Lemma in_Fs y E: y ∈ Fs E -> exists x, F x = y /\ x ∈ Vars__de E.