proposition: Lemma lexico3_2 : forall x1 x2 x3 y1 y2 y3, x1 = y1 -> R2 x2 y2 -> lexico3 R1 R2 R3 (x1,x2,x3) (y1,y2,y3).
proof: Proof. Proof using. intro. left. right~. Qed.
imports: TLC(LibTactics LibLogic LibBool LibLogic LibProd LibSum)