(* extension to cards and stacks *)
Definition f (P : SRS) : BSRS :=
map f_c P.

Lemma tau2_f A : tau2 (f A) = f_s (tau2 A).
Fixpoint g_s' (x : string bool) (n : nat) : string nat :=
match x with
| nil => nil
| true :: x' => g_s' x' (S n)
| false :: x' => n :: g_s' x' 0
end.