(** ** Enumerability *)
Fixpoint L_PCP n : list (BSRS * (string bool * string bool)) :=
match n with
| 0 => []
| S n => L_PCP n
++ [ (C, (x, y)) | (C, x, y) ∈ (L_T BSRS n × L_T (string bool) n × L_T (string bool) n), (x/y) el C ]
++ [ (C, (x ++ u, y ++ v)) | ( (C, (u,v)), x, y) ∈ (L_PCP n × L_T (string bool) n × L_T (string bool) n), (x,y) el C ]
end.

Lemma enum_PCP' :
enum (fun '(C, (u, v)) => @derivable bool C u v) L_PCP.