#[local] Ltac list_auto :=
      repeat lazymatch goal with
      | |- ?x = ?x =>
          reflexivity
      | H : ?xs = ?xs ++ _ |- _ =>
          rewrite <-(app_nil_r xs) in H at 1
      | H : ?xs ++ _ = ?xs |- _ =>
          symmetry in H
      | H : ?xs ++ _ = ?xs ++ _ |- _ =>
          apply app_inv_head in H
      | H : _ :: _ = _ :: _ |- _ =>
          injection H; intros; clear H; subst
      | H : [] = _ :: _ |- _ =>
          inversion H
      | H : cmp ?x ?x = Lt |- _ =>
          rewrite (proj2 (Hcmp _ _) eq_refl) in H; discriminate
      | H : cmp ?x ?x = Gt |- _ =>
          rewrite (proj2 (Hcmp _ _) eq_refl) in H; discriminate
      | H1 : ?p1 ++ _ :: _ = ?p2 ++ _ :: _,
        H2 : ?p2 ++ _ :: _ = ?p1 ++ _ :: _ |- _ =>
          symmetry in H2
      | H1 : ?p1 ++ ?x1 :: ?xs1 = ?p2 ++ ?x2 :: ?xs2,
        H2 : ?p1 ++ ?y1 :: ?ys1 = ?p2 ++ ?y2 :: ?ys2 |- _ =>
          assert (p1 = p2) as Hp;
          [ eapply (prefix_eq H1 H2); intros Heq; subst
          | subst; apply app_inv_head in H1, H2 ]
      | H : cmp ?x ?x = _ |- _ =>
          rewrite (proj2 (Hcmp _ _) eq_refl) in H; try discriminate H
      | H1 : cmp ?x1 ?x2 = _,
        H2 : cmp ?x1 ?x2 = _ |- _ =>
          rewrite H1 in H2; discriminate H2
      | Htrans : forall (x y z : A) (c : comparison), cmp x y = c -> cmp y z = c -> cmp x z = c,
        H1 : cmp ?x1 ?x2 = ?c,
        H2 : cmp ?x2 ?x3 = ?c |- _ =>
          pose proof (Htrans x1 x2 x3 c H1 H2); clear H1 H2
      | Hcmp_opp : (forall x y, cmp y x = CompOpp (cmp x y)),
        H1 : cmp ?x1 ?x2 = ?c, H2 : cmp ?x2 ?x1 = ?c |- _ =>
          rewrite Hcmp_opp, H2 in H1; simpl in H1; discriminate H1
      end.Inductive ListCompareSpec (xs ys : list A) : forall (c : comparison), Prop :=
      | ListCompareEq :
          xs = ys ->
          ListCompareSpec xs ys Eq
      | ListCompareShorter y ys' :
          ys = xs ++ y :: ys' ->
          ListCompareSpec xs ys Lt
      | ListCompareLonger x xs' :
          xs = ys ++ x :: xs' ->
          ListCompareSpec xs ys Gt
      | ListCompareLt prefix x xs' y ys' :
          xs = prefix ++ x :: xs' ->
          ys = prefix ++ y :: ys' ->
          cmp x y = Lt ->
          ListCompareSpec xs ys Lt
      | ListCompareGt prefix x xs' y ys' :
          xs = prefix ++ x :: xs' ->
          ys = prefix ++ y :: ys' ->
          cmp x y = Gt ->
          ListCompareSpec xs ys Gt.Inductive ListCompareSpec (xs ys : list A) : forall (c : comparison), Prop :=
      | ListCompareEq :
          xs = ys ->
          ListCompareSpec xs ys Eq
      | ListCompareShorter y ys' :
          ys = xs ++ y :: ys' ->
          ListCompareSpec xs ys Lt
      | ListCompareLonger x xs' :
          xs = ys ++ x :: xs' ->
          ListCompareSpec xs ys Gt
      | ListCompareLt prefix x xs' y ys' :
          xs = prefix ++ x :: xs' ->
          ys = prefix ++ y :: ys' ->
          cmp x y = Lt ->
          ListCompareSpec xs ys Lt
      | ListCompareGt prefix x xs' y ys' :
          xs = prefix ++ x :: xs' ->
          ys = prefix ++ y :: ys' ->
          cmp x y = Gt ->
          ListCompareSpec xs ys Gt.Inductive ListCompareSpec (xs ys : list A) : forall (c : comparison), Prop :=
      | ListCompareEq :
          xs = ys ->
          ListCompareSpec xs ys Eq
      | ListCompareShorter y ys' :
          ys = xs ++ y :: ys' ->
          ListCompareSpec xs ys Lt
      | ListCompareLonger x xs' :
          xs = ys ++ x :: xs' ->
          ListCompareSpec xs ys Gt
      | ListCompareLt prefix x xs' y ys' :
          xs = prefix ++ x :: xs' ->
          ys = prefix ++ y :: ys' ->
          cmp x y = Lt ->
          ListCompareSpec xs ys Lt
      | ListCompareGt prefix x xs' y ys' :
          xs = prefix ++ x :: xs' ->
          ys = prefix ++ y :: ys' ->
          cmp x y = Gt ->
          ListCompareSpec xs ys Gt.Inductive ListCompareSpec (xs ys : list A) : forall (c : comparison), Prop :=
      | ListCompareEq :
          xs = ys ->
          ListCompareSpec xs ys Eq
      | ListCompareShorter y ys' :
          ys = xs ++ y :: ys' ->
          ListCompareSpec xs ys Lt
      | ListCompareLonger x xs' :
          xs = ys ++ x :: xs' ->
          ListCompareSpec xs ys Gt
      | ListCompareLt prefix x xs' y ys' :
          xs = prefix ++ x :: xs' ->
          ys = prefix ++ y :: ys' ->
          cmp x y = Lt ->
          ListCompareSpec xs ys Lt
      | ListCompareGt prefix x xs' y ys' :
          xs = prefix ++ x :: xs' ->
          ys = prefix ++ y :: ys' ->
          cmp x y = Gt ->
          ListCompareSpec xs ys Gt.Inductive ListCompareSpec (xs ys : list A) : forall (c : comparison), Prop :=
      | ListCompareEq :
          xs = ys ->
          ListCompareSpec xs ys Eq
      | ListCompareShorter y ys' :
          ys = xs ++ y :: ys' ->
          ListCompareSpec xs ys Lt
      | ListCompareLonger x xs' :
          xs = ys ++ x :: xs' ->
          ListCompareSpec xs ys Gt
      | ListCompareLt prefix x xs' y ys' :
          xs = prefix ++ x :: xs' ->
          ys = prefix ++ y :: ys' ->
          cmp x y = Lt ->
          ListCompareSpec xs ys Lt
      | ListCompareGt prefix x xs' y ys' :
          xs = prefix ++ x :: xs' ->
          ys = prefix ++ y :: ys' ->
          cmp x y = Gt ->
          ListCompareSpec xs ys Gt.Inductive ListCompareSpec (xs ys : list A) : forall (c : comparison), Prop :=
      | ListCompareEq :
          xs = ys ->
          ListCompareSpec xs ys Eq
      | ListCompareShorter y ys' :
          ys = xs ++ y :: ys' ->
          ListCompareSpec xs ys Lt
      | ListCompareLonger x xs' :
          xs = ys ++ x :: xs' ->
          ListCompareSpec xs ys Gt
      | ListCompareLt prefix x xs' y ys' :
          xs = prefix ++ x :: xs' ->
          ys = prefix ++ y :: ys' ->
          cmp x y = Lt ->
          ListCompareSpec xs ys Lt
      | ListCompareGt prefix x xs' y ys' :
          xs = prefix ++ x :: xs' ->
          ys = prefix ++ y :: ys' ->
          cmp x y = Gt ->
          ListCompareSpec xs ys Gt.

Lemma list_compare_spec_complete (xs ys : list A) (c : comparison) : ListCompareSpec xs ys c -> list_compare xs ys = c.