proposition: Lemma demo_replaces : forall a b, a = b + b -> a + a + a = 3 * a.
proof: Proof. Proof using. intros. dup 3. replaces a with 3. demo. demo. replaces a at 2 with 5. demo. demo. replaces b at 2 with 4 in H. demo. demo. Qed.
imports: TLC(LibTactics) Coq.micromega.Lia