proposition: Lemma singles_two : forall x1 x2 v1 v2, (x1::x2::nil) ~* (v1::v2::nil) = (x2 ~ v2 & x1 ~ v1).
proof: Proof. Proof using. intros. rew_env_defs. simpl. rew_env_defs. fequals. Qed.
imports: TLC(LibTactics LibOption LibList LibProd LibLogic LibReflect)