proposition: Lemma get_opt_app : forall l1 l2 a, get_opt a (l1 ++ l2) = match get_opt a l1 with | None => get_opt a l2 | Some v => Some v end.
proof: Proof. Proof using. introv. induction l1 as [|(y,w) l1']; rew_list; simpl. { auto. } { case_if*. } Qed.
imports: TLC(LibTactics LibReflect)