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(********************************************************************)
(*                                                                  *)
(*  The Why3 Verification Platform   /   The Why3 Development Team  *)
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(*  Copyright 2010-2015   --   INRIA - CNRS - Paris-Sud University  *)
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(*                                                                  *)
(*  This software is distributed under the terms of the GNU Lesser  *)
(*  General Public License version 2.1, with the special exception  *)
(*  on linking described in file LICENSE.                           *)
(*                                                                  *)
(********************************************************************)
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open Stdlib
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open Ident
open Ty
open Term
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open Decl
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open Theory
open Mlw_ty
open Mlw_ty.T
open Mlw_expr

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let debug = Debug.register_info_flag "whyml_wp"
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  ~desc:"Print@ details@ of@ verification@ conditions@ generation."
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let no_track = Debug.register_flag "wp_no_track"
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  ~desc:"Do@ not@ remove@ redundant@ type@ invariant@ conditions@ from@ VCs."

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let no_eval = Debug.register_flag "wp_no_eval"
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  ~desc:"Do@ not@ simplify@ pattern@ matching@ on@ record@ datatypes@ in@ VCs."
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let lemma_label = Ident.create_label "why3:lemma"

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(** Marks *)
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let ts_mark = create_tysymbol (id_fresh "'mark") [] None
let ty_mark = ty_app ts_mark []

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let ity_mark = ity_pur ts_mark []
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let fresh_mark () = create_vsymbol (id_fresh "'mark") ty_mark
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let fs_at =
  let ty = ty_var (create_tvsymbol (id_fresh "a")) in
  create_lsymbol (id_fresh "at") [ty; ty_mark] (Some ty)

let fs_old =
  let ty = ty_var (create_tvsymbol (id_fresh "a")) in
  create_lsymbol (id_fresh "old") [ty] (Some ty)

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let mark_theory =
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  let uc = create_theory ~path:["why3";"Mark"] (id_fresh "Mark") in
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  let uc = add_ty_decl uc ts_mark in
  close_theory uc

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let th_mark_at =
  let uc = create_theory (id_fresh "WP builtins: at") in
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  let uc = use_export uc mark_theory in
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  let uc = add_param_decl uc fs_at in
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  close_theory uc

let th_mark_old =
  let uc = create_theory (id_fresh "WP builtins: old") in
  let uc = use_export uc th_mark_at in
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  let uc = add_param_decl uc fs_old in
  close_theory uc

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let fs_now = create_lsymbol (id_fresh "%now") [] (Some ty_mark)
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let t_now = fs_app fs_now [] ty_mark
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let e_now = e_ghost (e_lapp fs_now [] (ity_pur ts_mark []))
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(* [vs_old] appears in the postconditions given to the core API,
   which expects every vsymbol to be a pure part of a pvsymbol *)
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let pv_old = create_pvsymbol ~ghost:true (id_fresh "%old") ity_mark
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let vs_old = pv_old.pv_vs
let t_old  = t_var vs_old
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let t_at_old t = t_app fs_at [t; t_old] t.t_ty

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let ls_absurd = create_lsymbol (id_fresh "absurd") [] None
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let t_absurd  = t_label_add Split_goal.stop_split (ps_app ls_absurd [])
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let mk_t_if f = t_if f t_bool_true t_bool_false
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let to_term t = if t.t_ty = None then mk_t_if t else t
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(* any vs in post/xpost is either a pvsymbol or a fresh mark *)
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let ity_of_vs vs =
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  if Ty.ty_equal vs.vs_ty ty_mark then ity_mark else (restore_pv vs).pv_ity
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(* replace every occurrence of [old(t)] with [at(t,'old)] *)
let rec remove_old f = match f.t_node with
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  | Tapp (ls,[t]) when ls_equal ls fs_old -> t_at_old (remove_old t)
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  | _ -> t_map remove_old f

(* replace every occurrence of [at(t,'now)] with [t] *)
let rec remove_at f = match f.t_node with
  | Tapp (ls, [t; { t_node = Tapp (fs,[]) }])
    when ls_equal ls fs_at && ls_equal fs fs_now -> remove_at t
  | _ -> t_map remove_at f

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(* replace [at(t,'old)] with [at(t,lab)] everywhere in formula [f] *)
let old_mark lab t = t_subst_single vs_old (t_var lab) t

(* replace [at(t,lab)] with [at(t,'now)] everywhere in formula [f] *)
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let erase_mark lab t = t_subst_single lab t_now t

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(* retreat to the point of the current postcondition's ['old] *)
let backstep fn q xq =
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  let lab = fresh_mark () in
  let f = fn (old_mark lab q) (Mexn.map (old_mark lab) xq) in
  erase_mark lab f
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(** WP utilities *)
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let default_exn_post xs _ =
  let vs = create_vsymbol (id_fresh "result") (ty_of_ity xs.xs_ity) in
  create_post vs t_true

let default_post vty ef =
  let vs = create_vsymbol (id_fresh "result") (ty_of_vty vty) in
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  create_post vs t_true, Mexn.mapi default_exn_post ef.eff_raises
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let wp_label e f =
  let loc = if f.t_loc = None then e.e_loc else f.t_loc in
  let lab = Ident.Slab.union e.e_label f.t_label in
  t_label ?loc lab f

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let expl_pre       = Ident.create_label "expl:precondition"
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let expl_post      = Ident.create_label "expl:postcondition"
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let expl_xpost     = Ident.create_label "expl:exceptional postcondition"
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let expl_assume    = Ident.create_label "expl:assumption"
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let expl_assert    = Ident.create_label "expl:assertion"
let expl_check     = Ident.create_label "expl:check"
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let expl_absurd    = Ident.create_label "expl:unreachable point"
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let expl_type_inv  = Ident.create_label "expl:type invariant"
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let expl_loop_init = Ident.create_label "expl:loop invariant init"
let expl_loop_keep = Ident.create_label "expl:loop invariant preservation"
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let expl_loopvar   = Ident.create_label "expl:loop variant decrease"
let expl_variant   = Ident.create_label "expl:variant decrease"
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let lab_has_expl =
  let expl_regexp = Str.regexp "expl:\\(.*\\)" in
  Slab.exists
    (fun l ->
       Str.string_match expl_regexp l.lab_string 0)

let rec wp_expl l f =
  if lab_has_expl f.t_label then f
  else match f.t_node with
    | _ when Slab.mem Split_goal.stop_split f.t_label -> t_label_add l f
    | Tbinop (Tand,f1,f2) -> t_label_copy f (t_and (wp_expl l f1) (wp_expl l f2))
    | Teps _ -> t_label_add l f (* post-condition, push down later *)
    | _ -> f
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let wp_and ~sym f1 f2 =
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  if sym then t_and_simp f1 f2 else t_and_asym_simp f1 f2

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let wp_ands ~sym fl =
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  if sym then t_and_simp_l fl else t_and_asym_simp_l fl

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let wp_implies f1 f2 = t_implies_simp f1 f2
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let wp_let v t f = t_let_close_simp v t f

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let wp_forall vl f = t_forall_close_simp vl [] f

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let is_equality_for v f = match f.t_node with
  | Tapp (ps, [{ t_node = Tvar u }; t])
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    when ls_equal ps ps_equ && vs_equal u v && t_v_occurs v t = 0 ->
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      Some t
  | _ ->
      None

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let wp_forall_post v p f =
  (* we optimize for the case when a postcondition
     is of the form (... /\ result = t /\ ...) *)
  let rec down p = match p.t_node with
    | Tbinop (Tand,l,r) ->
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        let t, l, r =
          let t, l = down l in
          if t <> None then t, l, r else
            let t, r = down r in t, l, r
        in
        t, if t = None then p else t_label_copy p (t_and_simp l r)
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    | _ ->
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        let t = is_equality_for v p in
        t, if t = None then p else t_true
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  in
  if ty_equal v.vs_ty ty_unit then
    t_subst_single v t_void (wp_implies p f)
  else match down p with
    | Some t, p -> wp_let v t (wp_implies p f)
    | _ -> wp_forall [v] (wp_implies p f)
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let t_and_subst v t1 t2 =
  (* if [t1] defines variable [v], return [t2] with [v] replaced by its
     definition. Otherwise return [t1 /\ t2] *)
  match is_equality_for v t1 with
  | Some def -> t_subst_single v def t2
  | None -> t_and_simp t1 t2

let t_implies_subst v t1 t2 =
  (* if [t1] defines variable [v], return [t2] with [v] replaced by its
     definition. Otherwise return [t1 -> t2] *)
  match is_equality_for v t1 with
  | Some def -> t_subst_single v def t2
  | None -> t_implies_simp t1 t2

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let regs_of_writes eff = Sreg.union eff.eff_writes eff.eff_ghostw
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let exns_of_raises eff = Sexn.union eff.eff_raises eff.eff_ghostx
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let open_post q =
  let v, f = open_post q in
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  v, Slab.fold wp_expl q.t_label f
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let open_unit_post q =
  let v, q = open_post q in
  t_subst_single v t_void q

let create_unit_post =
  let v = create_vsymbol (id_fresh "void") ty_unit in
  fun q -> create_post v q

let vs_result e =
  create_vsymbol (id_fresh ?loc:e.e_loc "result") (ty_of_vty e.e_vty)

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(** WP state *)

type wp_env = {
  prog_known : Mlw_decl.known_map;
  pure_known : Decl.known_map;
  global_env : Env.env;
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  ps_int_le  : Term.lsymbol;
  ps_int_ge  : Term.lsymbol;
  ps_int_lt  : Term.lsymbol;
  ps_int_gt  : Term.lsymbol;
  fs_int_pl  : Term.lsymbol;
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  fs_int_mn  : Term.lsymbol;
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  letrec_var : variant list Mint.t;
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}
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let decrease_alg ?loc env old_t t =
  let oty = t_type old_t in
  let nty = t_type t in
  let quit () =
    Loc.errorm ?loc "no default order for %a" Pretty.print_term t in
  let ts = match oty with { ty_node = Tyapp (ts,_) } -> ts | _ -> quit () in
  let csl = Decl.find_constructors env.pure_known ts in
  if csl = [] then quit ();
  let sbs = ty_match Mtv.empty (ty_app ts (List.map ty_var ts.ts_args)) oty in
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  let add_arg fty acc =
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    let fty = ty_inst sbs fty in
    if ty_equal fty nty then
      let vs = create_vsymbol (id_fresh "f") nty in
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      pat_var vs, t_or_simp (t_equ (t_var vs) t) acc
    else pat_wild fty, acc in
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  let add_cs (cs,_) =
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    let pl, f = Lists.map_fold_right add_arg cs.ls_args t_false in
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    t_close_branch (pat_app cs pl oty) f in
  t_case old_t (List.map add_cs csl)

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let decrease_def ?loc env old_t t =
  if ty_equal (t_type old_t) ty_int && ty_equal (t_type t) ty_int
  then t_and (ps_app env.ps_int_le [t_nat_const 0;old_t])
             (ps_app env.ps_int_lt [t;old_t])
  else decrease_alg ?loc env old_t t
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let decrease loc lab env olds varl =
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  let rec decr olds varl = match olds, varl with
    | (old_t, Some old_r)::olds, (t, Some r)::varl
      when oty_equal old_t.t_ty t.t_ty && ls_equal old_r r ->
        let dt = ps_app r [t; old_t] in
        t_or_simp dt (t_and_simp (t_equ old_t t) (decr olds varl))
    | (old_t, None)::olds, (t, None)::varl
      when oty_equal old_t.t_ty t.t_ty ->
        let dt = decrease_def ?loc env old_t t in
        t_or_simp dt (t_and_simp (t_equ old_t t) (decr olds varl))
    | (old_t, None)::_, (t, None)::_ ->
        decrease_def ?loc env old_t t
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    | _ -> t_false
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  in
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  t_label ?loc lab (decr olds varl)
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let expl_variant = Slab.add Split_goal.stop_split (Slab.singleton expl_variant)
let expl_loopvar = Slab.add Split_goal.stop_split (Slab.singleton expl_loopvar)
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(** Reconstruct pure values after writes *)

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(* The counter-example model related data needed for creating new
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   variable. *)
type model_data = {
  md_append_to_model_trace : string;
    (* The string that will be appended to the end of "model_trace:" label.
       It is used to specify the reason why the variable is created. *)
  md_loc                   : Loc.position option;
    (* The location of the new variable. *)
  md_context_labels         : Slab.t option;
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    (* The labels of an element that represents the context in
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       that the variable is created.
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       Used in SPARK branch - the SPARK locations are kept in
       labels and when a new variable is created, these location
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       labels are copied from md_context_labels. *)
}

let create_model_data ?loc ?context_labels append_to_model_trace =
(* Creates new counter-example model related data.
   @param loc : the location of the new variable

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   @param context_labels : The labels of an element that represents the context in that
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   the variable is created.
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   Used in SPARK branch - the SPARK locations are kept in labels and when a new
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   variable is created, these location labels are copied from md_context_labels.

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   @param append_to_model_trace : The string that will be appended to the end of
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   "model_trace:" label. It is used to specify the reason why the variable is created. *)
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  {
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    md_append_to_model_trace = append_to_model_trace;
    md_loc = loc;
    md_context_labels = context_labels;
  }
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let mk_var id ty md =
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  let new_labels =
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    append_to_model_trace_label ~labels:id.id_label ~to_append:("@"^md.md_append_to_model_trace) in
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  create_vsymbol (id_fresh ~label:new_labels ?loc:md.md_loc id.id_string) ty
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(* replace "contemporary" variables with fresh ones *)
let rec subst_at_now now mvs t = match t.t_node with
  | Tvar vs when now ->
      begin try t_var (Mvs.find vs mvs) with Not_found -> t end
  | Tapp (ls, _) when ls_equal ls fs_old -> assert false
  | Tapp (ls, [_; mark]) when ls_equal ls fs_at ->
      let now = match mark.t_node with
        | Tvar vs when vs_equal vs vs_old -> assert false
        | Tapp (ls,[]) when ls_equal ls fs_now -> true
        | _ -> false in
      t_map (subst_at_now now mvs) t
  | Tlet _ | Tcase _ | Teps _ | Tquant _ ->
      (* do not open unless necessary *)
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      let mvs = Mvs.set_inter mvs (t_vars t) in
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      if Mvs.is_empty mvs then t else
      t_map (subst_at_now now mvs) t
  | _ ->
      t_map (subst_at_now now mvs) t

(* generic expansion of an algebraic type value *)
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let analyze_var fn_down fn_join lkm km vs ity =
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  let var_of_fd fd =
    create_vsymbol (id_fresh "y") (ty_of_ity fd.fd_ity) in
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  let branch (cs,fdl) =
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    let vl = List.map var_of_fd fdl in
    let pat = pat_app cs (List.map pat_var vl) vs.vs_ty in
    let t = fn_join cs (List.map2 fn_down vl fdl) vs.vs_ty in
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    t_close_branch pat t in
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  let csl = Mlw_decl.inst_constructors lkm km ity in
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  t_case_simp (t_var vs) (List.map branch csl)
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(* given a map of modified regions, construct the updated value of [vs] *)
let update_var env (mreg : vsymbol Mreg.t) vs =
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  let rec update vs { fd_ity = ity; fd_mut = mut } =
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    (* are we a mutable variable? *)
    let get_vs r = Mreg.find_def vs r mreg in
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    let vs = Opt.fold (fun _ -> get_vs) vs mut in
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    (* should we update our value further? *)
    let check_reg r _ = reg_occurs r ity.ity_vars in
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    if ity_immutable ity || not (Mreg.exists check_reg mreg) then t_var vs
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    else analyze_var update fs_app env.pure_known env.prog_known vs ity in
  update vs { fd_ity = ity_of_vs vs; fd_ghost = false; fd_mut = None }
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(* given a map of modified regions, update every affected variable in [f] *)
let update_term env (mreg : vsymbol Mreg.t) f =
  (* [vars] : modified variable -> updated value *)
  let update vs _ = match update_var env mreg vs with
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    | { t_node = Tvar nv } when vs_equal vs nv -> None
    | t -> Some t in
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  let vars = Mvs.mapi_filter update (t_vars f) in
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  (* [vv'] : modified variable -> fresh variable *)
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  let new_var vs _ = mk_var vs.vs_name vs.vs_ty (create_model_data ?loc:f.t_loc ~context_labels:f.t_label "model_quantify") in
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  let vv' = Mvs.mapi new_var vars in
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  (* update modified variables *)
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  let update v t f = wp_let (Mvs.find v vv') t f in
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  Mvs.fold update vars (subst_at_now true vv' f)

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let get_single_region_of_var vs =
  match (ity_of_vs vs).ity_node with
    | Ityapp (_,_,[r]) -> Some r
    | _ -> None

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(* look for a variable with a single region equal to [reg] *)
let var_of_region reg f =
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  let test acc vs =
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    match get_single_region_of_var vs with
    | Some r when reg_equal r reg -> Some vs
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    | _ -> acc in
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  t_v_fold test None f
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let quantify md env regs f =
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  (* mreg : modified region -> vs *)
  let get_var reg () =
    let ty = ty_of_ity reg.reg_ity in
    let id = match var_of_region reg f with
      | Some vs -> vs.vs_name
      | None -> reg.reg_name in
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    let md = match md.md_loc with
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      | None -> create_model_data
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	?loc:id.id_loc ~context_labels:id.id_label md.md_append_to_model_trace
      | _ -> md in
    mk_var id ty md in
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  let mreg = Mreg.mapi get_var regs in
  (* quantify over the modified resions *)
  let f = update_term env mreg f in
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  wp_forall (List.rev (Mreg.values mreg)) f
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(** Invariants *)

let get_invariant km t =
  let ty = t_type t in
  let ts = match ty.ty_node with
    | Tyapp (ts,_) -> ts
    | _ -> assert false in
  let rec find_td = function
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    | (its,_,inv) :: _ when ts_equal ts its.its_ts -> inv
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    | _ :: tdl -> find_td tdl
    | [] -> assert false in
  let pd = Mid.find ts.ts_name km in
  let inv = match pd.Mlw_decl.pd_node with
    | Mlw_decl.PDdata tdl -> find_td tdl
    | _ -> assert false in
  let sbs = Ty.ty_match Mtv.empty (t_type inv) ty in
  let u, p = open_post (t_ty_subst sbs Mvs.empty inv) in
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  wp_expl expl_type_inv (t_subst_single u t p)
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let ps_inv = Term.create_psymbol (id_fresh "inv")
  [ty_var (create_tvsymbol (id_fresh "a"))]

let full_invariant lkm km vs ity =
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  let rec update vs { fd_ity = ity } =
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    if not (ity_has_inv ity) then t_true else
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    (* what is our current invariant? *)
    let f = match ity.ity_node with
      | Ityapp (its,_,_) when its.its_inv ->
          if Debug.test_flag no_track
          then get_invariant km (t_var vs)
          else ps_app ps_inv [t_var vs]
      | _ -> t_true in
    (* what are our sub-invariants? *)
    let join _ fl _ = wp_ands ~sym:true fl in
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    let g = analyze_var update join lkm km vs ity in
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    (* put everything together *)
    wp_and ~sym:true f g
  in
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  update vs { fd_ity = ity; fd_ghost = false; fd_mut = None }
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(** Value tracking *)
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type point = int
type value = point list Mls.t (* constructor -> field list *)
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type state = {
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  st_km   : Mlw_decl.known_map;
  st_lkm  : Decl.known_map;
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  st_mem  : value Hint.t;
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  st_next : point ref;
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}

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(* dead code
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type names = point Mvs.t  (* variable -> point *)
type condition = lsymbol Mint.t (* point -> constructor *)
type lesson = condition list Mint.t (* point -> conditions for invariant *)
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*)
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let empty_state lkm km = {
  st_km   = km;
  st_lkm  = lkm;
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  st_mem  = Hint.create 5;
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  st_next = ref 0;
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}

let next_point state =
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  let res = !(state.st_next) in incr state.st_next; res
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let make_value state ty =
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  let get_p _ = next_point state in
  let new_cs cs = List.map get_p cs.ls_args in
  let add_cs m (cs,_) = Mls.add cs (new_cs cs) m in
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  let csl = match ty.ty_node with
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    | Tyapp (ts,_) -> Decl.find_constructors state.st_lkm ts
    | _ -> [] in
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  List.fold_left add_cs Mls.empty csl

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let match_point state ty p =
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  try Hint.find state.st_mem p with Not_found ->
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  let value = make_value state ty in
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  if not (Mls.is_empty value) then
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    Hint.replace state.st_mem p value;
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  value

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let rec open_pattern state names value p pat = match pat.pat_node with
  | Pwild -> names
  | Pvar vs -> Mvs.add vs p names
  | Papp (cs,patl) ->
      let add_pat names p pat =
        let value = match_point state pat.pat_ty p in
        open_pattern state names value p pat in
      List.fold_left2 add_pat names (Mls.find cs value) patl
  | Por _ ->
      let add_vs vs s = Mvs.add vs (next_point state) s in
      Svs.fold add_vs pat.pat_vars names
  | Pas (pat,vs) ->
      open_pattern state (Mvs.add vs p names) value p pat

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let rec point_of_term state names t = match t.t_node with
  | Tvar vs ->
      Mvs.find vs names
  | Tapp (ls, tl) ->
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      begin match Mid.find ls.ls_name state.st_lkm with
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        | { Decl.d_node = Decl.Ddata tdl } ->
            let is_cs (cs,_) = ls_equal ls cs in
            let is_cs (_,csl) = List.exists is_cs csl in
            if List.exists is_cs tdl
            then point_of_constructor state names ls tl
            else point_of_projection state names ls (List.hd tl)
        | _ -> next_point state
      end
  | Tlet (t1, bt) ->
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      let p1 = point_of_term state names t1 in
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      let v, t2 = t_open_bound bt in
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      let names = Mvs.add v p1 names in
      point_of_term state names t2
  | Tcase (t1,[br]) ->
      let pat, t2 = t_open_branch br in
      let p1 = point_of_term state names t1 in
      let value = match_point state pat.pat_ty p1 in
      let names = open_pattern state names value p1 pat in
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      point_of_term state names t2
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  | Tcase (t1,bl) ->
      (* we treat here the case of a value update: the value
         of each branch must be a distinct constructor *)
      let p = next_point state in
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      let ty = Opt.get t.t_ty in
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      let p1 = point_of_term state names t1 in
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      let value = match_point state (Opt.get t1.t_ty) p1 in
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      let branch acc br =
        let pat, t2 = t_open_branch br in
        let ls = match t2.t_node with
          | Tapp (ls,_) -> ls | _ -> raise Exit in
        let names = open_pattern state names value p1 pat in
        let p2 = point_of_term state names t2 in
        let v2 = match_point state ty p2 in
        Mls.add_new Exit ls (Mls.find_exn Exit ls v2) acc
      in
      begin try
        let value = List.fold_left branch Mls.empty bl in
        let value = Mls.set_union value (make_value state ty) in
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        Hint.replace state.st_mem p value
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      with Exit -> () end;
      p
  | Tconst _ | Tif _ | Teps _ -> next_point state
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  | Tquant _ | Tbinop _ | Tnot _ | Ttrue | Tfalse -> assert false

and point_of_constructor state names ls tl =
  let p = next_point state in
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  let pl = List.map (point_of_term state names) tl in
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  let value = make_value state (Opt.get ls.ls_value) in
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  let value = Mls.add ls pl value in
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  Hint.replace state.st_mem p value;
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  p

and point_of_projection state names ls t1 =
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  let ty = Opt.get t1.t_ty in
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  let csl = match ty.ty_node with
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    | Tyapp (ts,_) -> Decl.find_constructors state.st_lkm ts
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    | _ -> assert false in
  match csl with
    | [cs,pjl] ->
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        let p1 = point_of_term state names t1 in
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        let value = match_point state ty p1 in
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        let rec find_p pjl pl = match pjl, pl with
          | Some pj::_, p::_ when ls_equal ls pj -> p
          | _::pjl, _::pl -> find_p pjl pl
          | _ -> assert false in
        find_p pjl (Mls.find cs value)
    | _ -> next_point state (* more than one, can't choose *)

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let rec track_values state names lesson cond f = match f.t_node with
  | Tapp (ls, [t1]) when ls_equal ls ps_inv ->
      let p1 = point_of_term state names t1 in
      let condl = Mint.find_def [] p1 lesson in
      let contains c1 c2 = Mint.submap (fun _ -> ls_equal) c2 c1 in
      if List.exists (contains cond) condl then
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        lesson, t_label_copy f t_true
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      else
        let good c = not (contains c cond) in
        let condl = List.filter good condl in
        let l = Mint.add p1 (cond::condl) lesson in
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        l, t_label_copy f (get_invariant state.st_km t1)
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  | Tbinop (Timplies, f1, f2) ->
      let l, f1 = track_values state names lesson cond f1 in
      let _, f2 = track_values state names l cond f2 in
      lesson, t_label_copy f (t_implies_simp f1 f2)
  | Tbinop (Tand, f1, f2) ->
      let l, f1 = track_values state names lesson cond f1 in
      let l, f2 = track_values state names l cond f2 in
      l, t_label_copy f (t_and_simp f1 f2)
  | Tif (fc, f1, f2) ->
      let _, f1 = track_values state names lesson cond f1 in
      let _, f2 = track_values state names lesson cond f2 in
      lesson, t_label_copy f (t_if_simp fc f1 f2)
  | Tcase (t1, bl) ->
      let p1 = point_of_term state names t1 in
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      let value = match_point state (Opt.get t1.t_ty) p1 in
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      let is_pat_var = function
        | { pat_node = Pvar _ } -> true | _ -> false in
      let branch l br =
        let pat, f1, cb = t_open_branch_cb br in
        let learn, cond = match bl, pat.pat_node with
          | [_], _ -> true, cond (* one branch, can learn *)
          | _, Papp (cs, pl) when List.for_all is_pat_var pl ->
              (try true, Mint.add_new Exit p1 cs cond (* can learn *)
              with Exit -> false, cond) (* contradiction, cannot learn *)
          | _, _ -> false, cond (* complex pattern, will not learn *)
        in
        let names = open_pattern state names value p1 pat in
        let m, f1 = track_values state names lesson cond f1 in
        let l = if learn then m else l in
        l, cb pat f1
      in
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      let l, bl = Lists.map_fold_left branch lesson bl in
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      l, t_label_copy f (t_case_simp t1 bl)
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  | Tlet (t1, bf) ->
      let p1 = point_of_term state names t1 in
      let v, f1, cb = t_open_bound_cb bf in
      let names = Mvs.add v p1 names in
      let l, f1 = track_values state names lesson cond f1 in
      l, t_label_copy f (t_let_simp t1 (cb v f1))
  | Tquant (Tforall, qf) ->
      let vl, trl, f1, cb = t_open_quant_cb qf in
      let add_vs s vs = Mvs.add vs (next_point state) s in
      let names = List.fold_left add_vs names vl in
      let l, f1 = track_values state names lesson cond f1 in
      l, t_label_copy f (t_forall_simp (cb vl trl f1))
  | Tbinop ((Tor|Tiff),_,_) | Tquant (Texists,_)
  | Tapp _ | Tnot _ | Ttrue | Tfalse -> lesson, f
  | Tvar _ | Tconst _ | Teps _ -> assert false

let track_values lkm km f =
  let state = empty_state lkm km in
  let _, f = track_values state Mvs.empty Mint.empty Mint.empty f in
  f
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(** Weakest preconditions *)

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let rec wp_expr env e q xq =
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  let f = wp_desc env e q xq in
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  if Debug.test_flag debug then begin
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    Format.eprintf "@[--------@\n@[<hov 2>e = %a@]@\n" Mlw_pretty.print_expr e;
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    Format.eprintf "@[<hov 2>q = %a@]@\n" Pretty.print_term q;
    Format.eprintf "@[<hov 2>f = %a@]@\n----@]@." Pretty.print_term f;
  end;
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  f
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and wp_desc env e q xq = match e.e_node with
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  | Elogic t ->
      let v, q = open_post q in
      let t = wp_label e t in
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      (* NOTE: if you replace this t_subst by t_let or anything else,
         you must handle separately the case "let mark = 'now in ...",
         which requires 'now to be substituted for mark in q *)
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      if ty_equal v.vs_ty ty_mark then
        t_subst_single v (to_term t) q
      else
        t_let_close_simp v (to_term t) q
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  | Evalue pv ->
      let v, q = open_post q in
      let t = wp_label e (t_var pv.pv_vs) in
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      t_subst_single v t q
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  | Earrow _ ->
      let q = open_unit_post q in
      (* wp_label e *) q (* FIXME? *)
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  | Elet ({ let_sym = LetV v; let_expr = e1 }, e2)
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    when Opt.equal Loc.equal v.pv_vs.vs_name.id_loc e1.e_loc ->
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    (* we push the label down, past the implicitly inserted "let" *)
      let w = wp_expr env (e_label_copy e e2) q xq in
      let q = create_post v.pv_vs w in
      wp_expr env e1 q xq
  | Elet ({ let_sym = LetV v; let_expr = e1 }, e2) ->
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      let w = wp_expr env e2 q xq in
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      let q = create_post v.pv_vs w in
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      wp_label e (wp_expr env e1 q xq)
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  | Elet ({ let_sym = LetA _; let_expr = e1 }, e2) ->
      let w = wp_expr env e2 q xq in
      let q = create_unit_post w in
      wp_label e (wp_expr env e1 q xq)
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  | Erec (fdl, e1) ->
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      let eff = match e1.e_vty with
        | VTarrow _ -> None
        | VTvalue _ -> Some e1.e_effect in
      let fr = wp_rec_defn ?eff env fdl in
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      let fe = wp_expr env e1 q xq in
      let fr = wp_ands ~sym:true fr in
      wp_label e (wp_and ~sym:true fr fe)
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  | Eif (e1, e2, e3) ->
      let res = vs_result e1 in
      let test = t_equ (t_var res) t_bool_true in
      (* if both branches are pure, do not split *)
      let w =
        let get_term e = match e.e_node with
          | Elogic t -> to_term t
          | Evalue v -> t_var v.pv_vs
          | _ -> raise Exit in
        try
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          let r2 = wp_label e2 (get_term e2) in
          let r3 = wp_label e3 (get_term e3) in
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          let v, q = open_post q in
          t_subst_single v (t_if_simp test r2 r3) q
        with Exit ->
          let w2 = wp_expr env e2 q xq in
          let w3 = wp_expr env e3 q xq in
          t_if_simp test w2 w3
      in
      let q = create_post res w in
      wp_label e (wp_expr env e1 q xq)
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  (* optimization for the particular case let _ = e1 in e2 *)
  | Ecase (e1, [{ ppat_pattern = { pat_node = Term.Pwild }}, e2]) ->
      let w = wp_expr env e2 q xq in
      let q = create_post (vs_result e1) w in
      wp_label e (wp_expr env e1 q xq)
  (* optimization for the particular case let () = e1 in e2 *)
  | Ecase (e1, [{ ppat_pattern = { pat_node = Term.Papp (cs,[]) }}, e2])
    when ls_equal cs fs_void ->
      let w = wp_expr env e2 q xq in
      let q = create_unit_post w in
      wp_label e (wp_expr env e1 q xq)
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  | Ecase (e1, bl) ->
      let res = vs_result e1 in
      let branch ({ ppat_pattern = pat }, e) =
        t_close_branch pat (wp_expr env e q xq) in
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      let w = t_case_simp (t_var res) (List.map branch bl) in
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      let q = create_post res w in
      wp_label e (wp_expr env e1 q xq)
  | Eghost e1 ->
      wp_label e (wp_expr env e1 q xq)
  | Eraise (xs, e1) ->
      let q = try Mexn.find xs xq with
        Not_found -> assert false in
      wp_label e (wp_expr env e1 q xq)
  | Etry (e1, bl) ->
      let branch (xs,v,e) acc =
        let w = wp_expr env e q xq in
        let q = create_post v.pv_vs w in
        Mexn.add xs q acc in
      let xq = List.fold_right branch bl xq in
      wp_label e (wp_expr env e1 q xq)
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  | Eassert (Aassert, f) ->
      let q = open_unit_post q in
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      let f = wp_expl expl_assert f in
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      wp_and ~sym:false (wp_label e f) q
  | Eassert (Acheck, f) ->
      let q = open_unit_post q in
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      let f = wp_expl expl_check f in
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      wp_and ~sym:true (wp_label e f) q
  | Eassert (Aassume, f) ->
      let q = open_unit_post q in
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      let f = wp_expl expl_assume f in
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      wp_implies (wp_label e f) q
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  | Eabsurd ->
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      wp_label e (wp_expl expl_absurd t_absurd)
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  | Eany spec ->
      let p = wp_label e (wp_expl expl_pre spec.c_pre) in
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      let p = t_label ?loc:e.e_loc p.t_label p in
      (* TODO: propagate call labels into tyc.c_post *)
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      let w = wp_abstract (create_model_data "any") env spec.c_effect spec.c_post spec.c_xpost q xq in
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      wp_and ~sym:false p w
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  | Eapp (e1,_,spec) ->
      let p = wp_label e (wp_expl expl_pre spec.c_pre) in
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      let p = t_label ?loc:e.e_loc p.t_label p in
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      let d =
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        if spec.c_letrec = 0 || spec.c_variant = [] then t_true else
        let olds = Mint.find_def [] spec.c_letrec env.letrec_var in
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        if olds = [] then t_true (* we are out of letrec *) else
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        decrease e.e_loc expl_variant env olds spec.c_variant in
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      (* TODO: propagate call labels into tyc.c_post *)
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      let md = create_model_data ?loc:e1.e_loc ~context_labels:e1.e_label "call" in
      let w = wp_abstract md env spec.c_effect spec.c_post spec.c_xpost q xq in
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      let w = wp_and ~sym:true d (wp_and ~sym:false p w) in
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      let q = create_unit_post w in
      wp_expr env e1 q xq (* FIXME? should (wp_label e) rather be here? *)
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  | Eabstr (e1, spec) ->
      let p = wp_label e (wp_expl expl_pre spec.c_pre) in
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      (* every exception uncovered in spec is passed to xq *)
      let c_xq = Mexn.set_union spec.c_xpost xq in
      let w1 = backstep (wp_expr env e1) spec.c_post c_xq in
      (* so that now we don't need to prove these exceptions again *)
      let lost = Mexn.set_diff (exns_of_raises e1.e_effect) spec.c_xpost in
      let c_eff = Sexn.fold_left eff_remove_raise e1.e_effect lost in
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      let md = create_model_data ?loc:e1.e_loc ~context_labels:e1.e_label "abstract" in
      let w2 = wp_abstract md env c_eff spec.c_post spec.c_xpost q xq in
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      wp_and ~sym:false p (wp_and ~sym:true (wp_label e w1) w2)
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  | Eassign (pl, e1, reg, pv) ->
      (* if we create an intermediate variable npv to represent e1
         in the post-condition of the assign, the call to wp_abstract
         will have to update this variable separately (in addition to
         all existing variables in q that require update), creating
         duplication.  To avoid it, we try to detect whether the value
         of e1 can be represented by an existing pure term that can
         be reused in the post-condition. *)
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      let rec get_term d = match d.e_node with
        | Eghost e | Elet (_,e) | Erec (_,e) -> get_term e
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        | Evalue v -> vs_result e1, t_var v.pv_vs
        | Elogic t -> vs_result e1, t
        | _ ->
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            let ity = ity_of_expr e1 in
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            let id = id_fresh ?loc:e1.e_loc "o" in
            (* must be a pvsymbol or restore_pv will fail *)
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            let npv = create_pvsymbol id ~ghost:e1.e_ghost ity in
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            npv.pv_vs, t_var npv.pv_vs
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      in
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      let res, t = get_term e1 in
      let t = fs_app pl.pl_ls [t] pv.pv_vs.vs_ty in
      let c_q = create_unit_post (t_equ t (t_var pv.pv_vs)) in
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      let eff = eff_write eff_empty reg in
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      let md = create_model_data ?loc:e1.e_loc ~context_labels:e1.e_label "assign" in
      let w = wp_abstract md env eff c_q Mexn.empty q xq in
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      let q = create_post res w in
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      wp_label e (wp_expr env e1 q xq)
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  | Eloop (inv, varl, e1) ->
      (* TODO: what do we do about well-foundness? *)
      let i = wp_expl expl_loop_keep inv in
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      let olds = List.map (fun (t,r) -> t_at_old t , r) varl in
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      let i = if varl = [] then i else
        let d = decrease e.e_loc expl_loopvar env olds varl in
        wp_and ~sym:true i d in
      let q = create_unit_post i in
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      let w = backstep (wp_expr env e1) q xq in
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      let regs = regs_of_writes e1.e_effect in
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      let md = create_model_data ?loc:e1.e_loc ~context_labels:e1.e_label "loop" in
      let w = quantify md  env regs (wp_implies inv w) in
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      let i = wp_expl expl_loop_init inv in
      wp_label e (wp_and ~sym:true i w)
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  | Efor ({pv_vs = x}, ({pv_vs = v1}, d, {pv_vs = v2}), inv, e1) ->
      (* wp(for x = v1 to v2 do inv { I(x) } e1, Q, R) =
             v1 > v2  -> Q
         and v1 <= v2 ->     I(v1)
                         and forall S. forall i. v1 <= i <= v2 ->
                                                 I(i) -> wp(e1, I(i+1), R)
                                       and I(v2+1) -> Q *)
      let gt, le, incr = match d with
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        | Mlw_expr.To     -> env.ps_int_gt, env.ps_int_le, env.fs_int_pl
        | Mlw_expr.DownTo -> env.ps_int_lt, env.ps_int_ge, env.fs_int_mn
      in
      let one = t_nat_const 1 in
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      let v1_gt_v2 = ps_app gt [t_var v1; t_var v2] in
      let v1_le_v2 = ps_app le [t_var v1; t_var v2] in
      let q = open_unit_post q in
      let wp_init =
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        wp_expl expl_loop_init (t_subst_single x (t_var v1) inv) in
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      let wp_step =
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        let next = fs_app incr [t_var x; one] ty_int in
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        let post = wp_expl expl_loop_keep (t_subst_single x next inv) in
        wp_expr env e1 (create_unit_post post) xq in
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      let wp_last =
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        let v2pl1 = fs_app incr [t_var v2; one] ty_int in
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        wp_implies (t_subst_single x v2pl1 inv) q in
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      let md = create_model_data ?loc:e1.e_loc ~context_labels:e1.e_label "loop" in
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      let wp_good = wp_and ~sym:true
        wp_init