Definition e := (y0 ++ [#;$]) / [$].
Definition P := [d ; e] ++ R ++ [ ([#] / [#] )] ++ map (fun a => [a] / [a]) Sigma.

(* Specifications: *)
Lemma P_inv c :
c el P ->
c = d \/ c = e \/ c el R \/ c = ( [#] / [#] ) \/ exists a, a el Sigma /\ c = [a] / [a].
Lemma cards_subset x1 : x1 <<= Sigma -> cards x1 <<= P.
Lemma rew_subset u v : u / v el R -> u / v el P.
Fact SR_MPCP x :
x <<= Sigma -> x ≻* y0 -> exists A, A <<= P /\ tau1 A = x ++ [#] ++ tau2 A.