Section validity.
Context {ff : falsity_flag}.
Variable R : BSRS.
Fixpoint prep (x : string bool) (t : term) : term :=
match x with
| nil => t
| b::x => t_f b (prep x t)
end.
Definition enc s := (prep s t_e).

Lemma prep_app x y t :
prep (x ++ y) t = prep x (prep y t).