(* extension to cards and stacks *)
Definition f_c '(x,y) := (f_s x, f_s y).Definition f (P : SRS) : BSRS :=
  map f_c P.

(* Specifications: *)
Lemma tau1_f A : tau1 (f A) = f_s (tau1 A).
Lemma tau2_f A : tau2 (f A) = f_s (tau2 A).