(* extension to cards and stacks *)
Definition g (P : BSRS) : SRS :=
map g_c P.
(* Invariants *)

(* Specifications: *)
Lemma tau1_g A B : A <<= f B -> tau1 (g A) = g_s (tau1 A).
Lemma tau2_g A B : A <<= f B -> tau2 (g A) = g_s (tau2 A).
Lemma f_subset B A : A <<= B -> f A <<= f B.
Lemma f_g_subset B A : A <<= f B -> g A <<= B.
Lemma PCP_BPCP : PCP ⪯  BPCP.