proofmode.v

From iris.program_logic Require Export weakestpre total_weakestpre.
From iris.proofmode Require Import coq_tactics.
From iris.proofmode Require Export tactics.
From iris.heap_lang Require Export tactics lifting.
Set Default Proof Using "Type".
Import uPred.

Lemma tac_wp_expr_eval `{heapG Σ} Δ s E Φ e e' :
  (∀ (e'':=e'), e = e'') →
  envs_entails Δ (WP e' @ s; E {{ Φ }}) → envs_entails Δ (WP e @ s; E {{ Φ }}).
Proof. by intros ->. Qed.
Lemma tac_twp_expr_eval `{heapG Σ} Δ s E Φ e e' :
  (∀ (e'':=e'), e = e'') →
  envs_entails Δ (WP e' @ s; E [{ Φ }]) → envs_entails Δ (WP e @ s; E [{ Φ }]).
Proof. by intros ->. Qed.

Tactic Notation "wp_expr_eval" tactic(t) :=
  try (
    iStartProof;
    first [eapply tac_wp_expr_eval|eapply tac_twp_expr_eval];
      [let x := fresh in intros x; t; unfold x; reflexivity
      |]).

Ltac wp_expr_simpl := wp_expr_eval simpl.
Ltac wp_expr_simpl_subst := wp_expr_eval simpl_subst.

Lemma tac_wp_pure `{heapG Σ} Δ Δ' s E e1 e2 φ Φ :
  PureExec φ e1 e2 →
  φ →
  MaybeIntoLaterNEnvs 1 Δ Δ' →
  envs_entails Δ' (WP e2 @ s; E {{ Φ }}) →
  envs_entails Δ (WP e1 @ s; E {{ Φ }}).
Proof.
  rewrite /envs_entails=> ??? HΔ'. rewrite into_laterN_env_sound /=.
  rewrite HΔ' -lifting.wp_pure_step_later //.
Qed.
Lemma tac_twp_pure `{heapG Σ} Δ s E e1 e2 φ Φ :
  PureExec φ e1 e2 →
  φ →
  envs_entails Δ (WP e2 @ s; E [{ Φ }]) →
  envs_entails Δ (WP e1 @ s; E [{ Φ }]).
Proof.
  rewrite /envs_entails=> ?? ->. rewrite -total_lifting.twp_pure_step //.
Qed.

Lemma tac_wp_value `{heapG Σ} Δ s E Φ e v :
  IntoVal e v →
  envs_entails Δ (Φ v) → envs_entails Δ (WP e @ s; E {{ Φ }}).
Proof. rewrite /envs_entails=> ? ->. by apply wp_value. Qed.
Lemma tac_twp_value `{heapG Σ} Δ s E Φ e v :
  IntoVal e v →
  envs_entails Δ (Φ v) → envs_entails Δ (WP e @ s; E [{ Φ }]).
Proof. rewrite /envs_entails=> ? ->. by apply twp_value. Qed.

Ltac wp_value_head :=
  first [eapply tac_wp_value || eapply tac_twp_value];
    [apply _
    |iEval (lazy beta; simpl of_val)].

Tactic Notation "wp_pure" open_constr(efoc) :=
  iStartProof;
  lazymatch goal with
  | |- envs_entails _ (wp ?s ?E ?e ?Q) =>
    let e := eval simpl in e in
    reshape_expr e ltac:(fun K e' =>
      unify e' efoc;
      eapply (tac_wp_pure _ _ _ _ (fill K e'));
      [apply _                        (* PureExec *)
      |try fast_done                  (* The pure condition for PureExec *)
      |apply _                        (* IntoLaters *)
      |wp_expr_simpl_subst; try wp_value_head (* new goal *)
      ])
    || fail "wp_pure: cannot find" efoc "in" e "or" efoc "is not a redex"
  | |- envs_entails _ (twp ?s ?E ?e ?Q) =>
    let e := eval simpl in e in
    reshape_expr e ltac:(fun K e' =>
      unify e' efoc;
      eapply (tac_twp_pure _ _ _ (fill K e'));
      [apply _                        (* PureExec *)
      |try fast_done                  (* The pure condition for PureExec *)
      |wp_expr_simpl_subst; try wp_value_head (* new goal *)
      ])
    || fail "wp_pure: cannot find" efoc "in" e "or" efoc "is not a redex"
  | _ => fail "wp_pure: not a 'wp'"
  end.

Tactic Notation "wp_if" := wp_pure (If _ _ _).
Tactic Notation "wp_if_true" := wp_pure (If (Lit (LitBool true)) _ _).
Tactic Notation "wp_if_false" := wp_pure (If (Lit (LitBool false)) _ _).
Tactic Notation "wp_unop" := wp_pure (UnOp _ _).
Tactic Notation "wp_binop" := wp_pure (BinOp _ _ _).
Tactic Notation "wp_op" := wp_unop || wp_binop.
Tactic Notation "wp_rec" := wp_pure (App _ _).
Tactic Notation "wp_lam" := wp_rec.
Tactic Notation "wp_let" := wp_lam.
Tactic Notation "wp_seq" := wp_lam.
Tactic Notation "wp_proj" := wp_pure (Fst _) || wp_pure (Snd _).
Tactic Notation "wp_case" := wp_pure (Case _ _ _).
Tactic Notation "wp_match" := wp_case; wp_let.

Lemma tac_wp_bind `{heapG Σ} K Δ s E Φ e f :
  f = (λ e, fill K e) → (* as an eta expanded hypothesis so that we can `simpl` it *)
  envs_entails Δ (WP e @ s; E {{ v, WP f (of_val v) @ s; E {{ Φ }} }})%I →
  envs_entails Δ (WP fill K e @ s; E {{ Φ }}).
Proof. rewrite /envs_entails=> -> ->. by apply: wp_bind. Qed.
Lemma tac_twp_bind `{heapG Σ} K Δ s E Φ e f :
  f = (λ e, fill K e) → (* as an eta expanded hypothesis so that we can `simpl` it *)
  envs_entails Δ (WP e @ s; E [{ v, WP f (of_val v) @ s; E [{ Φ }] }])%I →
  envs_entails Δ (WP fill K e @ s; E [{ Φ }]).
Proof. rewrite /envs_entails=> -> ->. by apply: twp_bind. Qed.

Ltac wp_bind_core K :=
  lazymatch eval hnf in K with
  | [] => idtac
  | _ => eapply (tac_wp_bind K); [simpl; reflexivity|lazy beta]
  end.
Ltac twp_bind_core K :=
  lazymatch eval hnf in K with
  | [] => idtac
  | _ => eapply (tac_twp_bind K); [simpl; reflexivity|lazy beta]
  end.

Tactic Notation "wp_bind" open_constr(efoc) :=
  iStartProof;
  lazymatch goal with
  | |- envs_entails _ (wp ?s ?E ?e ?Q) =>
    reshape_expr e ltac:(fun K e' => unify e' efoc; wp_bind_core K)
    || fail "wp_bind: cannot find" efoc "in" e
  | |- envs_entails _ (twp ?s ?E ?e ?Q) =>
    reshape_expr e ltac:(fun K e' => unify e' efoc; twp_bind_core K)
    || fail "wp_bind: cannot find" efoc "in" e
  | _ => fail "wp_bind: not a 'wp'"
  end.

(** Heap tactics *)
Section heap.
Context `{heapG Σ}.
Implicit Types P Q : iProp Σ.
Implicit Types Φ : val → iProp Σ.
Implicit Types Δ : envs (iResUR Σ).

Lemma tac_wp_alloc Δ Δ' s E j K e v Φ :
  IntoVal e v →
  MaybeIntoLaterNEnvs 1 Δ Δ' →
  (∀ l, ∃ Δ'',
    envs_app false (Esnoc Enil j (l ↦ v)) Δ' = Some Δ'' ∧
    envs_entails Δ'' (WP fill K (Lit (LitLoc l)) @ s; E {{ Φ }})) →
  envs_entails Δ (WP fill K (Alloc e) @ s; E {{ Φ }}).
Proof.
  rewrite /envs_entails=> ?? HΔ.
  rewrite -wp_bind. eapply wand_apply; first exact: wp_alloc.
  rewrite left_id into_laterN_env_sound; apply later_mono, forall_intro=> l.
  destruct (HΔ l) as (Δ''&?&HΔ'). rewrite envs_app_sound //; simpl.
  by rewrite right_id HΔ'.
Qed.
Lemma tac_twp_alloc Δ s E j K e v Φ :
  IntoVal e v →
  (∀ l, ∃ Δ',
    envs_app false (Esnoc Enil j (l ↦ v)) Δ = Some Δ' ∧
    envs_entails Δ' (WP fill K (Lit (LitLoc l)) @ s; E [{ Φ }])) →
  envs_entails Δ (WP fill K (Alloc e) @ s; E [{ Φ }]).
Proof.
  rewrite /envs_entails=> ? HΔ.
  rewrite -twp_bind. eapply wand_apply; first exact: twp_alloc.
  rewrite left_id. apply forall_intro=> l.
  destruct (HΔ l) as (Δ'&?&HΔ'). rewrite envs_app_sound //; simpl.
  by rewrite right_id HΔ'.
Qed.

Lemma tac_wp_load Δ Δ' s E i K l q v Φ :
  MaybeIntoLaterNEnvs 1 Δ Δ' →
  envs_lookup i Δ' = Some (false, l ↦{q} v)%I →
  envs_entails Δ' (WP fill K (of_val v) @ s; E {{ Φ }}) →
  envs_entails Δ (WP fill K (Load (Lit (LitLoc l))) @ s; E {{ Φ }}).
Proof.
  rewrite /envs_entails=> ???.
  rewrite -wp_bind. eapply wand_apply; first exact: wp_load.
  rewrite into_laterN_env_sound -later_sep envs_lookup_split //; simpl.
  by apply later_mono, sep_mono_r, wand_mono.
Qed.
Lemma tac_twp_load Δ s E i K l q v Φ :
  envs_lookup i Δ = Some (false, l ↦{q} v)%I →
  envs_entails Δ (WP fill K (of_val v) @ s; E [{ Φ }]) →
  envs_entails Δ (WP fill K (Load (Lit (LitLoc l))) @ s; E [{ Φ }]).
Proof.
  rewrite /envs_entails=> ??.
  rewrite -twp_bind. eapply wand_apply; first exact: twp_load.
  rewrite envs_lookup_split //; simpl.
  by apply sep_mono_r, wand_mono.
Qed.

Lemma tac_wp_store Δ Δ' Δ'' s E i K l v e v' Φ :
  IntoVal e v' →
  MaybeIntoLaterNEnvs 1 Δ Δ' →
  envs_lookup i Δ' = Some (false, l ↦ v)%I →
  envs_simple_replace i false (Esnoc Enil i (l ↦ v')) Δ' = Some Δ'' →
  envs_entails Δ'' (WP fill K (Lit LitUnit) @ s; E {{ Φ }}) →
  envs_entails Δ (WP fill K (Store (Lit (LitLoc l)) e) @ s; E {{ Φ }}).
Proof.
  rewrite /envs_entails=> ?????.
  rewrite -wp_bind. eapply wand_apply; first by eapply wp_store.
  rewrite into_laterN_env_sound -later_sep envs_simple_replace_sound //; simpl.
  rewrite right_id. by apply later_mono, sep_mono_r, wand_mono.
Qed.
Lemma tac_twp_store Δ Δ' s E i K l v e v' Φ :
  IntoVal e v' →
  envs_lookup i Δ = Some (false, l ↦ v)%I →
  envs_simple_replace i false (Esnoc Enil i (l ↦ v')) Δ = Some Δ' →
  envs_entails Δ' (WP fill K (Lit LitUnit) @ s; E [{ Φ }]) →
  envs_entails Δ (WP fill K (Store (Lit (LitLoc l)) e) @ s; E [{ Φ }]).
Proof.
  intros. rewrite -twp_bind. eapply wand_apply; first by eapply twp_store.
  rewrite envs_simple_replace_sound //; simpl.
  rewrite right_id. by apply sep_mono_r, wand_mono.
Qed.

Lemma tac_wp_cas_fail Δ Δ' s E i K l q v e1 v1 e2 Φ :
  IntoVal e1 v1 → AsVal e2 →
  MaybeIntoLaterNEnvs 1 Δ Δ' →
  envs_lookup i Δ' = Some (false, l ↦{q} v)%I → v ≠ v1 →
  envs_entails Δ' (WP fill K (Lit (LitBool false)) @ s; E {{ Φ }}) →
  envs_entails Δ (WP fill K (CAS (Lit (LitLoc l)) e1 e2) @ s; E {{ Φ }}).
Proof.
  rewrite /envs_entails=> ??????.
  rewrite -wp_bind. eapply wand_apply; first exact: wp_cas_fail.
  rewrite into_laterN_env_sound -later_sep envs_lookup_split //; simpl.
  by apply later_mono, sep_mono_r, wand_mono.
Qed.
Lemma tac_twp_cas_fail Δ s E i K l q v e1 v1 e2 Φ :
  IntoVal e1 v1 → AsVal e2 →
  envs_lookup i Δ = Some (false, l ↦{q} v)%I → v ≠ v1 →
  envs_entails Δ (WP fill K (Lit (LitBool false)) @ s; E [{ Φ }]) →
  envs_entails Δ (WP fill K (CAS (Lit (LitLoc l)) e1 e2) @ s; E [{ Φ }]).
Proof.
  intros. rewrite -twp_bind. eapply wand_apply; first exact: twp_cas_fail.
  rewrite envs_lookup_split //; simpl. by apply sep_mono_r, wand_mono.
Qed.

Lemma tac_wp_cas_suc Δ Δ' Δ'' s E i K l v e1 v1 e2 v2 Φ :
  IntoVal e1 v1 → IntoVal e2 v2 →
  MaybeIntoLaterNEnvs 1 Δ Δ' →
  envs_lookup i Δ' = Some (false, l ↦ v)%I → v = v1 →
  envs_simple_replace i false (Esnoc Enil i (l ↦ v2)) Δ' = Some Δ'' →
  envs_entails Δ'' (WP fill K (Lit (LitBool true)) @ s; E {{ Φ }}) →
  envs_entails Δ (WP fill K (CAS (Lit (LitLoc l)) e1 e2) @ s; E {{ Φ }}).
Proof.
  rewrite /envs_entails=> ???????; subst.
  rewrite -wp_bind. eapply wand_apply; first exact: wp_cas_suc.
  rewrite into_laterN_env_sound -later_sep envs_simple_replace_sound //; simpl.
  rewrite right_id. by apply later_mono, sep_mono_r, wand_mono.
Qed.
Lemma tac_twp_cas_suc Δ Δ' s E i K l v e1 v1 e2 v2 Φ :
  IntoVal e1 v1 → IntoVal e2 v2 →
  envs_lookup i Δ = Some (false, l ↦ v)%I → v = v1 →
  envs_simple_replace i false (Esnoc Enil i (l ↦ v2)) Δ = Some Δ' →
  envs_entails Δ' (WP fill K (Lit (LitBool true)) @ s; E [{ Φ }]) →
  envs_entails Δ (WP fill K (CAS (Lit (LitLoc l)) e1 e2) @ s; E [{ Φ }]).
Proof.
  intros; subst. rewrite -twp_bind. eapply wand_apply; first exact: twp_cas_suc.
  rewrite envs_simple_replace_sound //; simpl.
  rewrite right_id. by apply sep_mono_r, wand_mono.
Qed.

Lemma tac_wp_faa Δ Δ' Δ'' s E i K l i1 e2 i2 Φ :
  IntoVal e2 (LitV (LitInt i2)) →
  MaybeIntoLaterNEnvs 1 Δ Δ' →
  envs_lookup i Δ' = Some (false, l ↦ LitV (LitInt i1))%I →
  envs_simple_replace i false (Esnoc Enil i (l ↦ LitV (LitInt (i1 + i2)))) Δ' = Some Δ'' →
  envs_entails Δ'' (WP fill K (Lit (LitInt i1)) @ s; E {{ Φ }}) →
  envs_entails Δ (WP fill K (FAA (Lit (LitLoc l)) e2) @ s; E {{ Φ }}).
Proof.
  rewrite /envs_entails=> ?????; subst.
  rewrite -wp_bind. eapply wand_apply; first exact: (wp_faa _ _ _ i1 _ i2).
  rewrite into_laterN_env_sound -later_sep envs_simple_replace_sound //; simpl.
  rewrite right_id. by apply later_mono, sep_mono_r, wand_mono.
Qed.
Lemma tac_twp_faa Δ Δ' s E i K l i1 e2 i2 Φ :
  IntoVal e2 (LitV (LitInt i2)) →
  envs_lookup i Δ = Some (false, l ↦ LitV (LitInt i1))%I →
  envs_simple_replace i false (Esnoc Enil i (l ↦ LitV (LitInt (i1 + i2)))) Δ = Some Δ' →
  envs_entails Δ' (WP fill K (Lit (LitInt i1)) @ s; E [{ Φ }]) →
  envs_entails Δ (WP fill K (FAA (Lit (LitLoc l)) e2) @ s; E [{ Φ }]).
Proof.
  rewrite /envs_entails=> ????; subst.
  rewrite -twp_bind. eapply wand_apply; first exact: (twp_faa _ _ _ i1 _ i2).
  rewrite envs_simple_replace_sound //; simpl.
  rewrite right_id. by apply sep_mono_r, wand_mono.
Qed.
End heap.

Tactic Notation "wp_apply" open_constr(lem) :=
  iPoseProofCore lem as false true (fun H =>
    lazymatch goal with
    | |- envs_entails _ (wp ?s ?E ?e ?Q) =>
      reshape_expr e ltac:(fun K e' =>
        wp_bind_core K; iApplyHyp H; try iNext; wp_expr_simpl) ||
      lazymatch iTypeOf H with
      | Some (_,?P) => fail "wp_apply: cannot apply" P
      end
    | |- envs_entails _ (twp ?s ?E ?e ?Q) =>
      reshape_expr e ltac:(fun K e' =>
        twp_bind_core K; iApplyHyp H; wp_expr_simpl) ||
      lazymatch iTypeOf H with
      | Some (_,?P) => fail "wp_apply: cannot apply" P
      end
    | _ => fail "wp_apply: not a 'wp'"
    end).

Tactic Notation "wp_alloc" ident(l) "as" constr(H) :=
  let finish _ :=
    first [intros l | fail 1 "wp_alloc:" l "not fresh"];
      eexists; split;
        [env_cbv; reflexivity || fail "wp_alloc:" H "not fresh"
        |wp_expr_simpl; try wp_value_head] in
  iStartProof;
  lazymatch goal with
  | |- envs_entails _ (wp ?s ?E ?e ?Q) =>
    first
      [reshape_expr e ltac:(fun K e' =>
         eapply (tac_wp_alloc _ _ _ _ H K); [apply _|..])
      |fail 1 "wp_alloc: cannot find 'Alloc' in" e];
    [apply _
    |finish ()]
  | |- envs_entails _ (twp ?s ?E ?e ?Q) =>
    first
      [reshape_expr e ltac:(fun K e' =>
         eapply (tac_twp_alloc _ _ _ H K); [apply _|..])
      |fail 1 "wp_alloc: cannot find 'Alloc' in" e];
    finish ()
  | _ => fail "wp_alloc: not a 'wp'"
  end.

Tactic Notation "wp_alloc" ident(l) :=
  let H := iFresh in wp_alloc l as H.

Tactic Notation "wp_load" :=
  let solve_mapsto _ :=
    let l := match goal with |- _ = Some (_, (?l ↦{_} _)%I) => l end in
    iAssumptionCore || fail "wp_load: cannot find" l "↦ ?" in
  iStartProof;
  lazymatch goal with
  | |- envs_entails _ (wp ?s ?E ?e ?Q) =>
    first
      [reshape_expr e ltac:(fun K e' => eapply (tac_wp_load _ _ _ _ _ K))
      |fail 1 "wp_load: cannot find 'Load' in" e];
    [apply _
    |solve_mapsto ()
    |wp_expr_simpl; try wp_value_head]
  | |- envs_entails _ (twp ?s ?E ?e ?Q) =>
    first
      [reshape_expr e ltac:(fun K e' => eapply (tac_twp_load _ _ _ _ K))
      |fail 1 "wp_load: cannot find 'Load' in" e];
    [solve_mapsto ()
    |wp_expr_simpl; try wp_value_head]
  | _ => fail "wp_load: not a 'wp'"
  end.

Tactic Notation "wp_store" :=
  let solve_mapsto _ :=
    let l := match goal with |- _ = Some (_, (?l ↦{_} _)%I) => l end in
    iAssumptionCore || fail "wp_store: cannot find" l "↦ ?" in
  let finish _ :=
    wp_expr_simpl; try first [wp_pure (Seq (Lit LitUnit) _)|wp_value_head] in
  iStartProof;
  lazymatch goal with
  | |- envs_entails _ (wp ?s ?E ?e ?Q) =>
    first
      [reshape_expr e ltac:(fun K e' =>
         eapply (tac_wp_store _ _ _ _ _ _ K); [apply _|..])
      |fail 1 "wp_store: cannot find 'Store' in" e];
    [apply _
    |solve_mapsto ()
    |env_cbv; reflexivity
    |finish ()]
  | |- envs_entails _ (twp ?s ?E ?e ?Q) =>
    first
      [reshape_expr e ltac:(fun K e' =>
         eapply (tac_twp_store _ _ _ _ _ K); [apply _|..])
      |fail 1 "wp_store: cannot find 'Store' in" e];
    [solve_mapsto ()
    |env_cbv; reflexivity
    |finish ()]
  | _ => fail "wp_store: not a 'wp'"
  end.

Tactic Notation "wp_cas_fail" :=
  let solve_mapsto _ :=
    let l := match goal with |- _ = Some (_, (?l ↦{_} _)%I) => l end in
    iAssumptionCore || fail "wp_cas_fail: cannot find" l "↦ ?" in
  iStartProof;
  lazymatch goal with
  | |- envs_entails _ (wp ?s ?E ?e ?Q) =>
    first
      [reshape_expr e ltac:(fun K e' =>
         eapply (tac_wp_cas_fail _ _ _ _ _ K); [apply _|apply _|..])
      |fail 1 "wp_cas_fail: cannot find 'CAS' in" e];
    [apply _
    |solve_mapsto ()
    |try congruence
    |simpl; try wp_value_head]
  | |- envs_entails _ (twp ?s ?E ?e ?Q) =>
    first
      [reshape_expr e ltac:(fun K e' =>
         eapply (tac_twp_cas_fail _ _ _ _ K); [apply _|apply _|..])
      |fail 1 "wp_cas_fail: cannot find 'CAS' in" e];
    [solve_mapsto ()
    |try congruence
    |wp_expr_simpl; try wp_value_head]
  | _ => fail "wp_cas_fail: not a 'wp'"
  end.

Tactic Notation "wp_cas_suc" :=
  let solve_mapsto _ :=
    let l := match goal with |- _ = Some (_, (?l ↦{_} _)%I) => l end in
    iAssumptionCore || fail "wp_cas_suc: cannot find" l "↦ ?" in
  iStartProof;
  lazymatch goal with
  | |- envs_entails _ (wp ?s ?E ?e ?Q) =>
    first
      [reshape_expr e ltac:(fun K e' =>
         eapply (tac_wp_cas_suc _ _ _ _ _ _ K); [apply _|apply _|..])
      |fail 1 "wp_cas_suc: cannot find 'CAS' in" e];
    [apply _
    |solve_mapsto ()
    |try congruence
    |env_cbv; reflexivity
    |simpl; try wp_value_head]
  | |- envs_entails _ (twp ?E ?e ?Q) =>
    first
      [reshape_expr e ltac:(fun K e' =>
         eapply (tac_twp_cas_suc _ _ _ _ _ K); [apply _|apply _|..])
      |fail 1 "wp_cas_suc: cannot find 'CAS' in" e];
    [solve_mapsto ()
    |try congruence
    |env_cbv; reflexivity
    |wp_expr_simpl; try wp_value_head]
  | _ => fail "wp_cas_suc: not a 'wp'"
  end.

Tactic Notation "wp_faa" :=
  let solve_mapsto _ :=
    let l := match goal with |- _ = Some (_, (?l ↦{_} _)%I) => l end in
    iAssumptionCore || fail "wp_cas_suc: cannot find" l "↦ ?" in
  iStartProof;
  lazymatch goal with
  | |- envs_entails _ (wp ?s ?E ?e ?Q) =>
    first
      [reshape_expr e ltac:(fun K e' =>
         eapply (tac_wp_faa _ _ _ _ _ _ K); [apply _|..])
      |fail 1 "wp_faa: cannot find 'CAS' in" e];
    [apply _
    |solve_mapsto ()
    |env_cbv; reflexivity
    |wp_expr_simpl; try wp_value_head]
  | |- envs_entails _ (twp ?s ?E ?e ?Q) =>
    first
      [reshape_expr e ltac:(fun K e' =>
         eapply (tac_twp_faa _ _ _ _ _ K); [apply _|..])
      |fail 1 "wp_faa: cannot find 'CAS' in" e];
    [solve_mapsto ()
    |env_cbv; reflexivity
    |wp_expr_simpl; try wp_value_head]
  | _ => fail "wp_faa: not a 'wp'"
  end.