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(* Copyright (c) 2012-2015, Robbert Krebbers. *)
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(* This file is distributed under the terms of the BSD license. *)
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(** This file collects general purpose tactics that are used throughout
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the development. *)
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From Coq Require Import Omega.
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From Coq Require Export Lia.
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From stdpp Require Export decidable.
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Lemma f_equal_dep {A B} (f g :  x : A, B x) x : f = g  f x = g x.
Proof. intros ->; reflexivity. Qed.
Lemma f_equal_help {A B} (f g : A  B) x y : f = g  x = y  f x = g y.
Proof. intros -> ->; reflexivity. Qed.
Ltac f_equal :=
  let rec go :=
    match goal with
    | _ => reflexivity
    | _ => apply f_equal_help; [go|try reflexivity]
    | |- ?f ?x = ?g ?x => apply (f_equal_dep f g); go
    end in
  try go.

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(** We declare hint databases [f_equal], [congruence] and [lia] and containing
solely the tactic corresponding to its name. These hint database are useful in
to be combined in combination with other hint database. *)
Hint Extern 998 (_ = _) => f_equal : f_equal.
Hint Extern 999 => congruence : congruence.
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Hint Extern 1000 => lia : lia.
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Hint Extern 1000 => omega : omega.
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Hint Extern 1001 => progress subst : subst. (** backtracking on this one will
be very bad, so use with care! *)
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(** The tactic [intuition] expands to [intuition auto with *] by default. This
is rather efficient when having big hint databases, or expensive [Hint Extern]
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declarations as the ones above. *)
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Tactic Notation "intuition" := intuition auto.

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(* [done] can get slow as it calls "trivial". [fast_done] can solve way less
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   goals, but it will also always finish quickly.
   We do 'reflexivity' last because for goals of the form ?x = y, if
   we have x = y in the context, we will typically want to use the
   assumption and not reflexivity *)
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Ltac fast_done :=
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  solve
    [ eassumption
    | symmetry; eassumption
    | apply not_symmetry; eassumption
    | reflexivity ].
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Tactic Notation "fast_by" tactic(tac) :=
  tac; fast_done.

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(** A slightly modified version of Ssreflect's finishing tactic [done]. It
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also performs [reflexivity] and uses symmetry of negated equalities. Compared
to Ssreflect's [done], it does not compute the goal's [hnf] so as to avoid
unfolding setoid equalities. Note that this tactic performs much better than
Coq's [easy] tactic as it does not perform [inversion]. *)
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Ltac done :=
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  solve
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  [ repeat first
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    [ fast_done
    | solve [trivial]
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    (* All the tactics below will introduce themselves anyway, or make no sense
       for goals of product type. So this is a good place for us to do it. *)
    | progress intros
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    | solve [symmetry; trivial]
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    | solve [apply not_symmetry; trivial]
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    | discriminate
    | contradiction
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    | split
    | match goal with H : ¬_ |- _ => case H; clear H; done end
    ]
  ].
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Tactic Notation "by" tactic(tac) :=
  tac; done.

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(** Aliases for trans and etrans that are easier to type *)
Tactic Notation "trans" constr(A) := transitivity A.
Tactic Notation "etrans" := etransitivity.

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(** Tactics for splitting conjunctions:

- [split_and] : split the goal if is syntactically of the shape [_ ∧ _]
- [split_ands?] : split the goal repeatedly (perhaps zero times) while it is
  of the shape [_ ∧ _].
- [split_ands!] : works similarly, but at least one split should succeed. In
  order to do so, it will head normalize the goal first to possibly expose a
  conjunction.

Note that [split_and] differs from [split] by only splitting conjunctions. The
[split] tactic splits any inductive with one constructor. *)
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Tactic Notation "split_and" :=
  match goal with
  | |- _  _ => split
  | |- Is_true (_ && _) => apply andb_True; split
  end.
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Tactic Notation "split_and" "?" := repeat split_and.
Tactic Notation "split_and" "!" := hnf; split_and; split_and?.
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Tactic Notation "destruct_and" "?" :=
  repeat match goal with
  | H : False |- _ => destruct H
  | H : _  _ |- _ => destruct H
  | H : Is_true (bool_decide _) |- _ => apply (bool_decide_unpack _) in H
  | H : Is_true (_ && _) |- _ => apply andb_True in H; destruct H
  end.
Tactic Notation "destruct_and" "!" := progress (destruct_and?).

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(** The tactic [case_match] destructs an arbitrary match in the conclusion or
assumptions, and generates a corresponding equality. This tactic is best used
together with the [repeat] tactical. *)
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Ltac case_match :=
  match goal with
  | H : context [ match ?x with _ => _ end ] |- _ => destruct x eqn:?
  | |- context [ match ?x with _ => _ end ] => destruct x eqn:?
  end.

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(** The tactic [unless T by tac_fail] succeeds if [T] is not provable by
the tactic [tac_fail]. *)
Tactic Notation "unless" constr(T) "by" tactic3(tac_fail) :=
  first [assert T by tac_fail; fail 1 | idtac].
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(** The tactic [repeat_on_hyps tac] repeatedly applies [tac] in unspecified
order on all hypotheses until it cannot be applied to any hypothesis anymore. *)
Tactic Notation "repeat_on_hyps" tactic3(tac) :=
  repeat match goal with H : _ |- _ => progress tac H end.

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(** The tactic [clear dependent H1 ... Hn] clears the hypotheses [Hi] and
their dependencies. *)
Tactic Notation "clear" "dependent" hyp(H1) hyp(H2) :=
  clear dependent H1; clear dependent H2.
Tactic Notation "clear" "dependent" hyp(H1) hyp(H2) hyp(H3) :=
  clear dependent H1 H2; clear dependent H3.
Tactic Notation "clear" "dependent" hyp(H1) hyp(H2) hyp(H3) hyp(H4) :=
  clear dependent H1 H2 H3; clear dependent H4.
Tactic Notation "clear" "dependent" hyp(H1) hyp(H2) hyp(H3) hyp(H4)
  hyp(H5) := clear dependent H1 H2 H3 H4; clear dependent H5.
Tactic Notation "clear" "dependent" hyp(H1) hyp(H2) hyp(H3) hyp(H4) hyp(H5)
  hyp (H6) := clear dependent H1 H2 H3 H4 H5; clear dependent H6.
Tactic Notation "clear" "dependent" hyp(H1) hyp(H2) hyp(H3) hyp(H4) hyp(H5)
  hyp (H6) hyp(H7) := clear dependent H1 H2 H3 H4 H5 H6; clear dependent H7.
Tactic Notation "clear" "dependent" hyp(H1) hyp(H2) hyp(H3) hyp(H4) hyp(H5)
  hyp (H6) hyp(H7) hyp(H8) :=
  clear dependent H1 H2 H3 H4 H5 H6 H7; clear dependent H8.
Tactic Notation "clear" "dependent" hyp(H1) hyp(H2) hyp(H3) hyp(H4) hyp(H5)
  hyp (H6) hyp(H7) hyp(H8) hyp(H9) :=
  clear dependent H1 H2 H3 H4 H5 H6 H7 H8; clear dependent H9.
Tactic Notation "clear" "dependent" hyp(H1) hyp(H2) hyp(H3) hyp(H4) hyp(H5)
  hyp (H6) hyp(H7) hyp(H8) hyp(H9) hyp(H10) :=
  clear dependent H1 H2 H3 H4 H5 H6 H7 H8 H9; clear dependent H10.

(** The tactic [is_non_dependent H] determines whether the goal's conclusion or
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hypotheses depend on [H]. *)
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Tactic Notation "is_non_dependent" constr(H) :=
  match goal with
  | _ : context [ H ] |- _ => fail 1
  | |- context [ H ] => fail 1
  | _ => idtac
  end.

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(** The tactic [var_eq x y] fails if [x] and [y] are unequal, and [var_neq]
does the converse. *)
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Ltac var_eq x1 x2 := match x1 with x2 => idtac | _ => fail 1 end.
Ltac var_neq x1 x2 := match x1 with x2 => fail 1 | _ => idtac end.

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(** Operational type class projections in recursive calls are not folded back
appropriately by [simpl]. The tactic [csimpl] uses the [fold_classes] tactics
to refold recursive calls of [fmap], [mbind], [omap] and [alter]. A
self-contained example explaining the problem can be found in the following
Coq-club message:

https://sympa.inria.fr/sympa/arc/coq-club/2012-10/msg00147.html *)
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Ltac fold_classes :=
  repeat match goal with
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  | |- context [ ?F ] =>
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    progress match type of F with
    | FMap _ =>
       change F with (@fmap _ F);
       repeat change (@fmap _ (@fmap _ F)) with (@fmap _ F)
    | MBind _ =>
       change F with (@mbind _ F);
       repeat change (@mbind _ (@mbind _ F)) with (@mbind _ F)
    | OMap _ =>
       change F with (@omap _ F);
       repeat change (@omap _ (@omap _ F)) with (@omap _ F)
    | Alter _ _ _ =>
       change F with (@alter _ _ _ F);
       repeat change (@alter _ _ _ (@alter _ _ _ F)) with (@alter _ _ _ F)
    end
  end.
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Ltac fold_classes_hyps H :=
  repeat match type of H with
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  | context [ ?F ] =>
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    progress match type of F with
    | FMap _ =>
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       change F with (@fmap _ F) in H;
       repeat change (@fmap _ (@fmap _ F)) with (@fmap _ F) in H
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    | MBind _ =>
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       change F with (@mbind _ F) in H;
       repeat change (@mbind _ (@mbind _ F)) with (@mbind _ F) in H
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    | OMap _ =>
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       change F with (@omap _ F) in H;
       repeat change (@omap _ (@omap _ F)) with (@omap _ F) in H
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    | Alter _ _ _ =>
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       change F with (@alter _ _ _ F) in H;
       repeat change (@alter _ _ _ (@alter _ _ _ F)) with (@alter _ _ _ F) in H
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    end
  end.
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Tactic Notation "csimpl" "in" hyp(H) :=
  try (progress simpl in H; fold_classes_hyps H).
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Tactic Notation "csimpl" := try (progress simpl; fold_classes).
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Tactic Notation "csimpl" "in" "*" :=
  repeat_on_hyps (fun H => csimpl in H); csimpl.
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(** The tactic [simplify_eq] repeatedly substitutes, discriminates,
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and injects equalities, and tries to contradict impossible inequalities. *)
Tactic Notation "simplify_eq" := repeat
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  match goal with
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  | H : _  _ |- _ => by destruct H
  | H : _ = _  False |- _ => by destruct H
  | H : ?x = _ |- _ => subst x
  | H : _ = ?x |- _ => subst x
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  | H : _ = _ |- _ => discriminate H
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  | H : _  _ |- _ => apply leibniz_equiv in H
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  | H : ?f _ = ?f _ |- _ => apply (inj f) in H
  | H : ?f _ _ = ?f _ _ |- _ => apply (inj2 f) in H; destruct H
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    (* before [injection] to circumvent bug #2939 in some situations *)
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  | H : ?f _ = ?f _ |- _ => progress injection H as H
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    (* first hyp will be named [H], subsequent hyps will be given fresh names *)
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  | H : ?f _ _ = ?f _ _ |- _ => progress injection H as H
  | H : ?f _ _ _ = ?f _ _ _ |- _ => progress injection H as H
  | H : ?f _ _ _ _ = ?f _ _ _ _ |- _ => progress injection H as H
  | H : ?f _ _ _ _ _ = ?f _ _ _ _ _ |- _ => progress injection H as H
  | H : ?f _ _ _ _ _ _ = ?f _ _ _ _ _ _ |- _ => progress injection H as H
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  | H : ?x = ?x |- _ => clear H
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    (* unclear how to generalize the below *)
  | H1 : ?o = Some ?x, H2 : ?o = Some ?y |- _ =>
    assert (y = x) by congruence; clear H2
  | H1 : ?o = Some ?x, H2 : ?o = None |- _ => congruence
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  | H : @existT ?A _ _ _ = existT _ _ |- _ =>
     apply (Eqdep_dec.inj_pair2_eq_dec _ (decide_rel (@eq A))) in H
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  end.
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Tactic Notation "simplify_eq" "/=" :=
  repeat (progress csimpl in * || simplify_eq).
Tactic Notation "f_equal" "/=" := csimpl in *; f_equal.
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Ltac setoid_subst_aux R x :=
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  match goal with
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  | H : R x ?y |- _ =>
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     is_var x;
     try match y with x _ => fail 2 end;
     repeat match goal with
     | |- context [ x ] => setoid_rewrite H
     | H' : context [ x ] |- _ =>
        try match H' with H => fail 2 end;
        setoid_rewrite H in H'
     end;
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     clear x H
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  end.
Ltac setoid_subst :=
  repeat match goal with
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  | _ => progress simplify_eq/=
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  | H : @equiv ?A ?e ?x _ |- _ => setoid_subst_aux (@equiv A e) x
  | H : @equiv ?A ?e _ ?x |- _ => symmetry in H; setoid_subst_aux (@equiv A e) x
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  end.

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(** f_equiv works on goals of the form [f _ = f _], for any relation and any
number of arguments. It looks for an appropriate [Proper] instance, and applies
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it. The tactic is somewhat limited, since it cannot be used to backtrack on
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the Proper instances that has been found. To that end, we try to avoid the
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trivial instance in which the resulting goals have an [eq]. More generally,
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we try to "maintain" the relation of the current goal. For example,
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when having [Proper (equiv ==> dist) f] and [Proper (dist ==> dist) f], it will
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favor the second because the relation (dist) stays the same. *)
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Ltac f_equiv :=
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  match goal with
  | _ => reflexivity
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  (* We support matches on both sides, *if* they concern the same variable, or
     variables in some relation. *)
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  | |- ?R (match ?x with _ => _ end) (match ?x with _ => _ end) =>
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    destruct x
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  | H : ?R ?x ?y |- ?R2 (match ?x with _ => _ end) (match ?y with _ => _ end) =>
     destruct H
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  (* First assume that the arguments need the same relation as the result *)
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  | |- ?R (?f ?x) (?f _) => apply (_ : Proper (R ==> R) f)
  (* For the case in which R is polymorphic, or an operational type class,
  like equiv. *)
  | |- (?R _) (?f ?x) (?f _) => apply (_ : Proper (R _ ==> _) f)
  | |- (?R _ _) (?f ?x) (?f _) => apply (_ : Proper (R _ _ ==> _) f)
  | |- (?R _ _ _) (?f ?x) (?f _) => apply (_ : Proper (R _ _ _ ==> _) f)
  | |- (?R _) (?f ?x ?y) (?f _ _) => apply (_ : Proper (R _ ==> R _ ==> _) f)
  | |- (?R _ _) (?f ?x ?y) (?f _ _) => apply (_ : Proper (R _ _ ==> R _ _ ==> _) f)
  | |- (?R _ _ _) (?f ?x ?y) (?f _ _) => apply (_ : Proper (R _ _ _ ==> R _ _ _ ==> _) f)
  | |- (?R _ _ _ _) (?f ?x ?y) (?f _ _) => apply (_ : Proper (R _ _ _ _ ==> R _ _ _ _ ==> _) f)
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  (* Next, try to infer the relation. Unfortunately, there is an instance
     of Proper for (eq ==> _), which will always be matched. *)
  (* TODO: Can we exclude that instance? *)
  (* TODO: If some of the arguments are the same, we could also
     query for "pointwise_relation"'s. But that leads to a combinatorial
     explosion about which arguments are and which are not the same. *)
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  | |- ?R (?f ?x) (?f _) => apply (_ : Proper (_ ==> R) f)
  | |- ?R (?f ?x ?y) (?f _ _) => apply (_ : Proper (_ ==> _ ==> R) f)
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   (* In case the function symbol differs, but the arguments are the same,
      maybe we have a pointwise_relation in our context. *)
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  | H : pointwise_relation _ ?R ?f ?g |- ?R (?f ?x) (?g ?x) => apply H
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  end.
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(** auto_proper solves goals of the form "f _ = f _", for any relation and any
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    number of arguments, by repeatedly applying f_equiv and handling trivial cases.
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    If it cannot solve an equality, it will leave that to the user. *)
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Ltac auto_equiv :=
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  (* Deal with "pointwise_relation" *)
  repeat lazymatch goal with
  | |- pointwise_relation _ _ _ _ => intros ?
  end;
  (* Normalize away equalities. *)
  simplify_eq;
  (* repeatedly apply congruence lemmas and use the equalities in the hypotheses. *)
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  try (f_equiv; fast_done || auto_equiv).
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(** solve_proper solves goals of the form "Proper (R1 ==> R2)", for any
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    number of relations. All the actual work is done by auto_equiv;
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    solve_proper just introduces the assumptions and unfolds the first
    head symbol. *)
Ltac solve_proper :=
  (* Introduce everything *)
  intros;
  repeat lazymatch goal with
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  | |- Proper _ _ => intros ???
  | |- (_ ==> _)%signature _ _ => intros ???
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  | |- pointwise_relation _ _ _ _ => intros ?
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  | |- ?R ?f _ => try let f' := constr:(λ x, f x) in intros ?
  end; simpl;
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  (* Unfold the head symbol, which is the one we are proving a new property about *)
  lazymatch goal with
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  | |- ?R (?f _ _ _ _ _ _ _ _) (?f _ _ _ _ _ _ _ _) => unfold f
  | |- ?R (?f _ _ _ _ _ _ _) (?f _ _ _ _ _ _ _) => unfold f
  | |- ?R (?f _ _ _ _ _ _) (?f _ _ _ _ _ _) => unfold f
  | |- ?R (?f _ _ _ _ _) (?f _ _ _ _ _) => unfold f
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  | |- ?R (?f _ _ _ _) (?f _ _ _ _) => unfold f
  | |- ?R (?f _ _ _) (?f _ _ _) => unfold f
  | |- ?R (?f _ _) (?f _ _) => unfold f
  | |- ?R (?f _) (?f _) => unfold f
  end;
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  solve [ auto_equiv ].
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(** The tactic [intros_revert tac] introduces all foralls/arrows, performs tac,
and then reverts them. *)
Ltac intros_revert tac :=
  lazymatch goal with
  | |-  _, _ => let H := fresh in intro H; intros_revert tac; revert H
  | |- _ => tac
  end.
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(** Given a tactic [tac2] generating a list of terms, [iter tac1 tac2]
runs [tac x] for each element [x] until [tac x] succeeds. If it does not
suceed for any element of the generated list, the whole tactic wil fail. *)
Tactic Notation "iter" tactic(tac) tactic(l) :=
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  let rec go l :=
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  match l with ?x :: ?l => tac x || go l end in go l.
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(** Given [H : A_1 → ... → A_n → B] (where each [A_i] is non-dependent), the
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tactic [feed tac H tac_by] creates a subgoal for each [A_i] and calls [tac p]
with the generated proof [p] of [B]. *)
Tactic Notation "feed" tactic(tac) constr(H) :=
  let rec go H :=
  let T := type of H in
  lazymatch eval hnf in T with
  | ?T1  ?T2 =>
    (* Use a separate counter for fresh names to make it more likely that
    the generated name is "fresh" with respect to those generated before
    calling the [feed] tactic. In particular, this hack makes sure that
    tactics like [let H' := fresh in feed (fun p => pose proof p as H') H] do
    not break. *)
    let HT1 := fresh "feed" in assert T1 as HT1;
      [| go (H HT1); clear HT1 ]
  | ?T1 => tac H
  end in go H.

(** The tactic [efeed tac H] is similar to [feed], but it also instantiates
dependent premises of [H] with evars. *)
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Tactic Notation "efeed" constr(H) "using" tactic3(tac) "by" tactic3 (bytac) :=
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  let rec go H :=
  let T := type of H in
  lazymatch eval hnf in T with
  | ?T1  ?T2 =>
    let HT1 := fresh "feed" in assert T1 as HT1;
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      [bytac | go (H HT1); clear HT1 ]
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  | ?T1  _ =>
    let e := fresh "feed" in evar (e:T1);
    let e' := eval unfold e in e in
    clear e; go (H e')
  | ?T1 => tac H
  end in go H.
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Tactic Notation "efeed" constr(H) "using" tactic3(tac) :=
  efeed H using tac by idtac.
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(** The following variants of [pose proof], [specialize], [inversion], and
[destruct], use the [feed] tactic before invoking the actual tactic. *)
Tactic Notation "feed" "pose" "proof" constr(H) "as" ident(H') :=
  feed (fun p => pose proof p as H') H.
Tactic Notation "feed" "pose" "proof" constr(H) :=
  feed (fun p => pose proof p) H.

Tactic Notation "efeed" "pose" "proof" constr(H) "as" ident(H') :=
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  efeed H using (fun p => pose proof p as H').
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Tactic Notation "efeed" "pose" "proof" constr(H) :=
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  efeed H using (fun p => pose proof p).
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Tactic Notation "feed" "specialize" hyp(H) :=
  feed (fun p => specialize p) H.
Tactic Notation "efeed" "specialize" hyp(H) :=
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  efeed H using (fun p => specialize p).
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Tactic Notation "feed" "inversion" constr(H) :=
  feed (fun p => let H':=fresh in pose proof p as H'; inversion H') H.
Tactic Notation "feed" "inversion" constr(H) "as" simple_intropattern(IP) :=
  feed (fun p => let H':=fresh in pose proof p as H'; inversion H' as IP) H.

Tactic Notation "feed" "destruct" constr(H) :=
  feed (fun p => let H':=fresh in pose proof p as H'; destruct H') H.
Tactic Notation "feed" "destruct" constr(H) "as" simple_intropattern(IP) :=
  feed (fun p => let H':=fresh in pose proof p as H'; destruct H' as IP) H.

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(** The block definitions are taken from [Coq.Program.Equality] and can be used
by tactics to separate their goal from hypotheses they generalize over. *)
Definition block {A : Type} (a : A) := a.

Ltac block_goal := match goal with [ |- ?T ] => change (block T) end.
Ltac unblock_goal := unfold block in *.

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(** The following tactic can be used to add support for patterns to tactic notation:
It will search for the first subterm of the goal matching [pat], and then call [tac]
with that subterm. *)
Ltac find_pat pat tac :=
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  match goal with
  |- context [?x] =>
      unify pat x with typeclass_instances;
      tryif tac x then idtac else fail 2
  end.
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(** Coq's [firstorder] tactic fails or loops on rather small goals already. In 
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particular, on those generated by the tactic [unfold_elem_ofs] which is used
to solve propositions on collections. The [naive_solver] tactic implements an
ad-hoc and incomplete [firstorder]-like solver using Ltac's backtracking
mechanism. The tactic suffers from the following limitations:
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- It might leave unresolved evars as Ltac provides no way to detect that.
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- To avoid the tactic becoming too slow, we allow a universally quantified
  hypothesis to be instantiated only once during each search path.
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- It does not perform backtracking on instantiation of universally quantified
  assumptions.

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We use a counter to make the search breath first. Breath first search ensures
that a minimal number of hypotheses is instantiated, and thus reduced the
posibility that an evar remains unresolved.

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Despite these limitations, it works much better than Coq's [firstorder] tactic
for the purposes of this development. This tactic either fails or proves the
goal. *)
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Lemma forall_and_distr (A : Type) (P Q : A  Prop) :
  ( x, P x  Q x)  ( x, P x)  ( x, Q x).
Proof. firstorder. Qed.

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(** The tactic [no_new_unsolved_evars tac] executes [tac] and fails if it
creates any new evars. This trick is by Jonathan Leivent, see:
https://coq.inria.fr/bugs/show_bug.cgi?id=3872 *)

Ltac no_new_unsolved_evars tac := exact ltac:(tac).

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Tactic Notation "naive_solver" tactic(tac) :=
  unfold iff, not in *;
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  repeat match goal with
  | H : context [ _, _  _ ] |- _ =>
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    repeat setoid_rewrite forall_and_distr in H; revert H
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  end;
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  let rec go n :=
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  repeat match goal with
  (**i intros *)
  | |-  _, _ => intro
  (**i simplification of assumptions *)
  | H : False |- _ => destruct H
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  | H : _  _ |- _ =>
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     (* Work around bug https://coq.inria.fr/bugs/show_bug.cgi?id=2901 *)
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     let H1 := fresh in let H2 := fresh in
     destruct H as [H1 H2]; try clear H
  | H :  _, _  |- _ =>
     let x := fresh in let Hx := fresh in
     destruct H as [x Hx]; try clear H
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  | H : ?P  ?Q, H2 : ?P |- _ => specialize (H H2)
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  | H : Is_true (bool_decide _) |- _ => apply (bool_decide_unpack _) in H
  | H : Is_true (_ && _) |- _ => apply andb_True in H; destruct H
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  (**i simplify and solve equalities *)
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  | |- _ => progress simplify_eq/=
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  (**i solve the goal *)
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  | |- _ => fast_done
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  (**i operations that generate more subgoals *)
  | |- _  _ => split
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  | |- Is_true (bool_decide _) => apply (bool_decide_pack _)
  | |- Is_true (_ && _) => apply andb_True; split
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  | H : _  _ |- _ =>
     let H1 := fresh in destruct H as [H1|H1]; try clear H
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  (**i solve the goal using the user supplied tactic *)
  | |- _ => solve [tac]
  end;
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  (**i use recursion to enable backtracking on the following clauses. *)
  match goal with
  (**i instantiation of the conclusion *)
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  | |-  x, _ => no_new_unsolved_evars ltac:(eexists; go n)
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  | |- _  _ => first [left; go n | right; go n]
  | _ =>
    (**i instantiations of assumptions. *)
    lazymatch n with
    | S ?n' =>
      (**i we give priority to assumptions that fit on the conclusion. *)
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      match goal with
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      | H : _  _ |- _ =>
        is_non_dependent H;
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        no_new_unsolved_evars
          ltac:(first [eapply H | efeed pose proof H]; clear H; go n')
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      end
    end
  end
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  in iter (fun n' => go n') (eval compute in (seq 1 6)).
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Tactic Notation "naive_solver" := naive_solver eauto.