proposition: Lemma derv_trans X R x y z : @derv X R x y -> derv R y z -> derv R x z.
proof: Proof. induction 1. - eauto. - intros. econstructor; eauto. Qed.
imports: List Undecidability.StringRewriting.PCSnf Undecidability.StringRewriting.SR Undecidability.StringRewriting.Util.Definitions