proposition: Lemma demo_rewrites_at : forall x y z, x = y + y -> x + x = z -> z + z = x + x + x + x.
proof: Proof. Proof using. introv E H. dup. rewrite E. demo. rewrites E at 2. rewrites E at 2 in H. rewrites <- H at 2. demo. Qed.
imports: TLC(LibTactics) Coq.micromega.Lia