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(** {1 A certified WP calculus} *)

(** {2 A simple imperative language with expressions, syntax and semantics} *)

theory ImpExpr

use import int.Int
use import int.MinMax
use import bool.Bool
use export list.List
use map.Map as IdMap

(** types and values *)

type datatype = TYunit | TYint | TYbool
type value = Vvoid | Vint int | Vbool bool

(** terms and formulas *)

type operator = Oplus | Ominus | Omult | Ole

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(** ident for mutable variables *)
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type mident

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axiom mident_decide :
  forall m1 m2: mident. m1 = m2 \/ m1 <> m2

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(** ident for immutable variables *)
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type ident = {| ident_index : int |}

(** Terms *)
type term_node =
  | Tvalue value
  | Tvar ident
  | Tderef mident
  | Tbin term operator term

with term = {| term_node : term_node;
               term_maxvar : int;
             |}

predicate var_occurs_in_term (x:ident) (t:term) =
  match t with
  | {| term_node = Tvalue _ |} -> false
  | {| term_node = Tvar i |} -> x=i
  | {| term_node = Tderef _ |} -> false
  | {| term_node = Tbin t1 _ t2 |} -> var_occurs_in_term x t1 \/ var_occurs_in_term x t2
  end

predicate term_inv (t:term) =
  forall x:ident. var_occurs_in_term x t -> x.ident_index <= t.term_maxvar

function mk_tvalue (v:value) : term =
   {| term_node = Tvalue v; term_maxvar = -1 |}

lemma mk_tvalue_inv :
   forall v:value. term_inv (mk_tvalue v)

function mk_tvar (i:ident) : term =
   {| term_node = Tvar i; term_maxvar = i.ident_index |}

lemma mk_tvar_inv :
   forall i:ident. term_inv (mk_tvar i)

function mk_tderef (r:mident) : term =
   {| term_node = Tderef r; term_maxvar = -1 |}

lemma mk_tderef_inv :
   forall r:mident. term_inv (mk_tderef r)

function mk_tbin (t1:term) (o:operator) (t2:term) : term =
   {| term_node = Tbin t1 o t2;
      term_maxvar = max t1.term_maxvar t2.term_maxvar |}

lemma mk_tbin_inv :
   forall t1 t2:term, o:operator. term_inv t1 /\ term_inv t2 ->
     term_inv (mk_tbin t1 o t2)


(** Formulas *)
type fmla =
  | Fterm term
  | Fand fmla fmla
  | Fnot fmla
  | Fimplies fmla fmla
  | Flet ident term fmla         (* let id = term in fmla *)
  | Fforall ident datatype fmla  (* forall id : ty, fmla *)

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(** Statements *)
type stmt =
  | Sskip
  | Sassign mident term
  | Sseq stmt stmt
  | Sif term stmt stmt
  | Sassert fmla
  | Swhile term fmla stmt  (* while cond invariant inv body *)
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lemma decide_is_skip:
  forall s:stmt. s = Sskip \/ s <> Sskip

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(** Typing *)

function type_value (v:value) : datatype =
    match v with
      | Vvoid  -> TYunit
      | Vint int ->  TYint
      | Vbool bool -> TYbool
end

inductive type_operator (op:operator) (ty1 ty2 ty: datatype) =
      | Type_plus : type_operator Oplus TYint TYint TYint
      | Type_minus : type_operator Ominus TYint TYint TYint
      | Type_mult : type_operator Omult TYint TYint TYint
      | Type_le : type_operator Ole TYint TYint TYbool

type type_stack = list (ident, datatype)  (* map local immutable variables to their type *)
function get_vartype (i:ident) (pi:type_stack) : datatype =
  match pi with
  | Nil -> TYunit
  | Cons (x,ty) r -> if x=i then ty else get_vartype i r
  end

type type_env = IdMap.map mident datatype  (* map global mutable variables to their type *)
function get_reftype (i:mident) (e:type_env) : datatype = IdMap.get e i

inductive type_term type_env type_stack term datatype =
  | Type_value :
      forall sigma: type_env, pi:type_stack, v:value, m:int.
	type_term sigma pi {| term_node = Tvalue v; term_maxvar = m |} (type_value v)
  | Type_var :
      forall sigma: type_env, pi:type_stack, v: ident, m:int, ty:datatype.
        (get_vartype v pi = ty) ->
        type_term sigma pi {| term_node = Tvar v ; term_maxvar = m |} ty
  | Type_deref :
      forall sigma: type_env, pi:type_stack, v: mident, m:int, ty:datatype.
        (get_reftype v sigma = ty) ->
        type_term sigma pi {| term_node = Tderef v; term_maxvar = m |} ty
  | Type_bin :
      forall sigma: type_env, pi:type_stack, t1 t2 : term, op:operator,
        m:int, ty1 ty2 ty:datatype.
        type_term sigma pi t1 ty1 ->
	type_term sigma pi t2 ty2 ->
	type_operator op ty1 ty2 ty ->
        type_term sigma pi {| term_node = Tbin t1 op t2; term_maxvar = m |} ty

inductive type_fmla type_env type_stack fmla =
  | Type_term :
      forall sigma: type_env, pi:type_stack, t:term.
	type_term sigma pi t TYbool ->
	type_fmla sigma pi (Fterm t)
  | Type_conj :
      forall sigma: type_env, pi:type_stack, f1 f2:fmla.
	type_fmla sigma pi f1 ->
        type_fmla sigma pi f2 ->
        type_fmla sigma pi (Fand f1 f2)
  | Type_neg :
      forall sigma: type_env, pi:type_stack, f:fmla.
	type_fmla sigma pi f ->
        type_fmla sigma pi (Fnot f)
  | Type_implies :
      forall sigma: type_env, pi:type_stack, f1 f2:fmla.
	type_fmla sigma pi f1 ->
        type_fmla sigma pi f2 ->
        type_fmla sigma pi (Fimplies f1 f2)
  | Type_let :
      forall sigma: type_env, pi:type_stack, x:ident, t:term, f:fmla, ty:datatype.
	type_term sigma pi t ty ->
        type_fmla sigma (Cons (x,ty) pi) f ->
        type_fmla sigma pi (Flet x t f)
  | Type_forall1 :
      forall sigma: type_env, pi:type_stack, x:ident, f:fmla.
        type_fmla sigma (Cons (x,TYint) pi) f ->
  	type_fmla sigma pi (Fforall x TYint f)
  | Type_forall2 :
      forall sigma: type_env, pi:type_stack, x:ident, f:fmla.
        type_fmla sigma (Cons (x,TYbool) pi) f ->
  	type_fmla sigma pi (Fforall x TYbool f)
  | Type_forall3:
      forall sigma: type_env, pi:type_stack, x:ident, f:fmla.
        type_fmla sigma (Cons (x,TYunit) pi) f ->
  	type_fmla sigma pi (Fforall x TYunit f)

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inductive type_stmt type_env type_stack stmt =
  | Type_skip :
      forall sigma: type_env, pi:type_stack.
	type_stmt sigma pi Sskip
  | Type_seq :
      forall sigma: type_env, pi:type_stack, s1 s2:stmt.
        type_stmt sigma pi s1 ->
	type_stmt sigma pi s2 ->
	type_stmt sigma pi (Sseq s1 s2)
  | Type_assigns :
      forall sigma: type_env, pi:type_stack, x:mident, t:term, ty:datatype.
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	(get_reftype x sigma = ty) ->
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        type_term sigma pi t ty ->
        type_stmt sigma pi (Sassign x t)
  | Type_if :
      forall sigma: type_env, pi:type_stack, t:term, s1 s2:stmt.
	type_term sigma pi t TYbool ->
	type_stmt sigma pi s1 ->
	type_stmt sigma pi s2 ->
    	type_stmt sigma pi (Sif t s1 s2)
  | Type_assert :
      forall sigma: type_env, pi:type_stack, p:fmla.
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	type_fmla sigma pi p ->
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    	type_stmt sigma pi (Sassert p)
  | Type_while :
      forall sigma: type_env, pi:type_stack, guard:term, body:stmt, inv:fmla.
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	type_fmla sigma pi inv ->
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        type_term sigma pi guard TYbool ->
        type_stmt sigma pi body ->
        type_stmt sigma pi (Swhile guard inv body) 
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(** Operational semantic *)
type env = IdMap.map mident value  (* map global mutable variables to their value *)
function get_env (i:mident) (e:env) : value = IdMap.get e i

type stack = list (ident, value)  (* map local immutable variables to their value *)
function get_stack (i:ident) (pi:stack) : value =
  match pi with
  | Nil -> Vvoid
  | Cons (x,v) r -> if x=i then v else get_stack i r
  end

lemma get_stack_eq:
  forall x:ident, v:value, r:stack.
    get_stack x (Cons (x,v) r) = v

lemma get_stack_neq:
  forall x i:ident, v:value, r:stack.
    x <> i -> get_stack i (Cons (x,v) r) = get_stack i r

(** semantics of formulas *)

function eval_bin (x:value) (op:operator) (y:value) : value =
  match x,y with
  | Vint x,Vint y ->
     match op with
     | Oplus -> Vint (x+y)
     | Ominus -> Vint (x-y)
     | Omult -> Vint (x*y)
     | Ole -> Vbool (if x <= y then True else False)
     end
  | _,_ -> Vvoid
  end

function eval_term (sigma:env) (pi:stack) (t:term) : value =
  match t with
  | {| term_node = Tvalue v |} -> v
  | {| term_node = Tvar id |} -> get_stack id pi
  | {| term_node = Tderef id |} -> get_env id sigma
  | {| term_node = Tbin t1 op t2 |} ->
     eval_bin (eval_term sigma pi t1) op (eval_term sigma pi t2)
  end

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lemma eval_bool_term:
  forall sigma:env, pi:stack, sigmat:type_env, pit:type_stack, t:term.
    type_term sigmat pit t TYbool ->
    (* TODO: compatibility sigma, sigmat and pi,pit *)
    exists b:bool.
      eval_term sigma pi t = Vbool b


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predicate eval_fmla (sigma:env) (pi:stack) (f:fmla) =
  match f with
  | Fterm t -> eval_term sigma pi t = Vbool True
  | Fand f1 f2 -> eval_fmla sigma pi f1 /\ eval_fmla sigma pi f2
  | Fnot f -> not (eval_fmla sigma pi f)
  | Fimplies f1 f2 -> eval_fmla sigma pi f1 -> eval_fmla sigma pi f2
  | Flet x t f ->
      eval_fmla sigma (Cons (x,eval_term sigma pi t) pi) f
  | Fforall x TYint f ->
     forall n:int. eval_fmla sigma (Cons (x,Vint n) pi) f
  | Fforall x TYbool f ->
     forall b:bool. eval_fmla sigma (Cons (x,Vbool b) pi) f
  | Fforall x TYunit f ->  eval_fmla sigma (Cons (x,Vvoid) pi) f
  end

(** substitution of a reference [r] by a logic variable [v]
   warning: proper behavior only guaranted if [v] is "fresh",
   i.e index(v) > term_maxvar(t) *)

function msubst_term (t:term) (r:mident) (v:ident) : term =
  match t with
  | {| term_node = Tvalue _ | Tvar _ |} -> t
  | {| term_node = Tderef x |} -> if r = x then mk_tvar v else t
  | {| term_node = Tbin t1 op t2 |} ->
      mk_tbin (msubst_term t1 r v) op (msubst_term t2 r v) 
  end

function subst_term (t:term) (r:ident) (v:ident) : term =
  match t with
  | {| term_node = Tvalue _ | Tderef _ |} -> t
  | {| term_node = Tvar x |} ->
      if r = x then mk_tvar v else t
  | {| term_node = Tbin t1 op t2 |} ->
     mk_tbin (subst_term t1 r v) op (subst_term t2 r v)
  end

(** [fresh_in_term id t] is true when [id] does not occur in [t] *)
predicate fresh_in_term (id:ident) (t:term) =
  id.ident_index > t.term_maxvar

lemma eval_msubst_term:
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  forall e:term, sigma:env, pi:stack, x:mident, v:ident.
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    fresh_in_term v e ->
    eval_term sigma pi (msubst_term e x v) =
    eval_term (IdMap.set sigma x (get_stack v pi)) pi e

lemma eval_subst_term:
  forall sigma:env, pi:stack, e:term, x:ident, v:ident.
    fresh_in_term v e ->
    eval_term sigma pi (subst_term e x v) =
    eval_term sigma (Cons (x, (get_stack v pi)) pi) e

lemma eval_term_change_free :
  forall t:term, sigma:env, pi:stack, id:ident, v:value.
    fresh_in_term id t ->
    eval_term sigma (Cons (id,v) pi) t = eval_term sigma pi t

predicate fresh_in_fmla (id:ident) (f:fmla) =
  match f with
  | Fterm e -> fresh_in_term id e
  | Fand f1 f2   | Fimplies f1 f2 ->
      fresh_in_fmla id f1 /\ fresh_in_fmla id f2
  | Fnot f -> fresh_in_fmla id f
  | Flet y t f -> id <> y /\ fresh_in_term id t /\ fresh_in_fmla id f
  | Fforall y ty f -> id <> y /\ fresh_in_fmla id f
  end

function subst (f:fmla) (x:ident) (v:ident) : fmla =
  match f with
  | Fterm e -> Fterm (subst_term e x v)
  | Fand f1 f2 -> Fand (subst f1 x v) (subst f2 x v)
  | Fnot f -> Fnot (subst f x v)
  | Fimplies f1 f2 -> Fimplies (subst f1 x v) (subst f2 x v)
  | Flet y t f -> Flet y (subst_term t x v) (subst f x v)
  | Fforall y ty f -> Fforall y ty (subst f x v)
  end

function msubst (f:fmla) (x:mident) (v:ident) : fmla =
  match f with
  | Fterm e -> Fterm (msubst_term e x v)
  | Fand f1 f2 -> Fand (msubst f1 x v) (msubst f2 x v)
  | Fnot f -> Fnot (msubst f x v)
  | Fimplies f1 f2 -> Fimplies (msubst f1 x v) (msubst f2 x v)
  | Flet y t f -> Flet y (msubst_term t x v) (msubst f x v)
  | Fforall y ty f -> Fforall y ty (msubst f x v)
  end

lemma subst_fresh :
  forall f:fmla, x:ident, v:ident.
   fresh_in_fmla x f -> subst f x v = f

lemma let_subst:
    forall t:term, f:fmla, x id':ident, id :mident.
    msubst (Flet x t f) id id' = Flet x (msubst_term t id id') (msubst f id id')

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(* Need it for monotonicity*)
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lemma eval_msubst:
  forall f:fmla, sigma:env, pi:stack, x:mident, v:ident.
    fresh_in_fmla v f ->
    (eval_fmla sigma pi (msubst f x v) <->
     eval_fmla (IdMap.set sigma x (get_stack v pi)) pi f)

lemma eval_subst:
  forall f:fmla, sigma:env, pi:stack, x:ident, v:ident.
    fresh_in_fmla v f ->
    (eval_fmla sigma pi (subst f x v) <->
     eval_fmla sigma (Cons(x, (get_stack v pi)) pi) f)

lemma eval_swap:
  forall f:fmla, sigma:env, pi:stack, id1 id2:ident, v1 v2:value.
    id1 <> id2 ->
    (eval_fmla sigma (Cons (id1,v1) (Cons (id2,v2) pi)) f <->
    eval_fmla sigma (Cons (id2,v2) (Cons (id1,v1) pi)) f)

lemma eval_same_var:
  forall f:fmla, sigma:env, pi:stack, id:ident, v1 v2:value.
    eval_fmla sigma (Cons (id,v1) (Cons (id,v2) pi)) f <->
    eval_fmla sigma (Cons (id,v1) pi) f

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  (* Need it for monotonicity*)
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lemma eval_change_free :
  forall f:fmla, sigma:env, pi:stack, id:ident, v:value.
    fresh_in_fmla id f ->
    (eval_fmla sigma (Cons (id,v) pi) f <-> eval_fmla sigma pi f)

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(** [valid_fmla f] is true when [f] is valid in any environment *)
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  predicate valid_fmla (p:fmla) = forall sigma:env, pi:stack. eval_fmla sigma pi p

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(* Not needed *)
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axiom msubst_implies :
forall p q:fmla.
  valid_fmla (Fimplies p q) ->
  forall sigma:env, pi:stack, x:mident, id:ident.
    fresh_in_fmla id (Fand p q) -> 
    eval_fmla sigma (Cons (id, (get_env x sigma)) pi) (Fimplies (msubst p x id) (msubst q x id)) 

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(** let id' = t in f[id <- id'] <=> let id = t in f*)
lemma let_equiv :
  forall id:ident, id':ident, t:term, f:fmla.
    forall sigma:env, pi:stack.
      fresh_in_fmla id' f ->
	eval_fmla sigma pi (Flet id' t (subst f id id'))
	 -> eval_fmla sigma pi (Flet id t f)

lemma let_implies :
  forall id:ident, t:term, p q:fmla.
    valid_fmla (Fimplies p q) ->
    valid_fmla (Fimplies (Flet id t p) (Flet id t q))

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predicate fresh_in_stmt (id:ident) (s:stmt) =
  match s with
  | Sskip -> true
  | Sseq s1 s2 -> fresh_in_stmt id s1 /\ fresh_in_stmt id s2
  | Sassign _ t -> fresh_in_term id t
  | Sif t s1 s2 -> fresh_in_term id t /\ fresh_in_stmt id s1 /\ fresh_in_stmt id s2
  | Sassert f -> fresh_in_fmla id f
  | Swhile cond inv body -> fresh_in_term id cond /\ fresh_in_fmla id inv /\ fresh_in_stmt id body
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  end


(** small-steps semantics for expressions *)

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inductive one_step env stack stmt env stack stmt =

  | one_step_assign :
      forall sigma sigma':env, pi:stack, x:mident, t:term.
        sigma' = IdMap.set sigma x (eval_term sigma pi t) ->
        one_step sigma pi (Sassign x t) sigma' pi Sskip

  | one_step_seq_noskip:
      forall sigma sigma':env, pi pi':stack, s1 s1' s2:stmt.
        one_step sigma pi s1 sigma' pi' s1' ->
          one_step sigma pi (Sseq s1 s2) sigma' pi' (Sseq s1' s2)

  | one_step_seq_skip:
      forall sigma:env, pi:stack, s:stmt.
        one_step sigma pi (Sseq Sskip s) sigma pi s
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  | one_step_if_true:
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      forall sigma:env, pi:stack, t:term, s1 s2:stmt.
        eval_term sigma pi t = Vbool True ->
        one_step sigma pi (Sif t s1 s2) sigma pi s1
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  | one_step_if_false:
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      forall sigma:env, pi:stack, t:term, s1 s2:stmt.
        eval_term sigma pi t = Vbool False ->
        one_step sigma pi (Sif t s1 s2) sigma pi s2
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  | one_step_assert:
      forall sigma:env, pi:stack, f:fmla.
        (* blocking semantics *)
        eval_fmla sigma pi f ->
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          one_step sigma pi (Sassert f) sigma pi Sskip
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  | one_step_while_true:
      forall sigma:env, pi:stack, cond:term, inv:fmla, body:stmt.
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        (* blocking semantics *)
        eval_fmla sigma pi inv ->
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        eval_term sigma pi cond = Vbool True ->
        one_step sigma pi (Swhile cond inv body) sigma pi
        (Sseq body (Swhile cond inv body))

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  | one_step_while_false:
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      forall sigma:env, pi:stack, cond:term, inv:fmla, body:stmt.
        (* blocking semantics *)
        eval_fmla sigma pi inv ->
        eval_term sigma pi cond = Vbool False ->
        one_step sigma pi (Swhile cond inv body) sigma pi Sskip
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 (** many steps of execution *)

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 inductive many_steps env stack stmt env stack stmt int =
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   | many_steps_refl:
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     forall sigma:env, pi:stack, s:stmt. many_steps sigma pi s sigma pi s 0
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   | many_steps_trans:
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     forall sigma1 sigma2 sigma3:env, pi1 pi2 pi3:stack, s1 s2 s3:stmt, n:int.
       one_step sigma1 pi1 s1 sigma2 pi2 s2 ->
       many_steps sigma2 pi2 s2 sigma3 pi3 s3 n ->
       many_steps sigma1 pi1 s1 sigma3 pi3 s3 (n+1)
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  lemma steps_non_neg:
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    forall sigma1 sigma2:env, pi1 pi2:stack, s1 s2:stmt, n:int.
      many_steps sigma1 pi1 s1 sigma2 pi2 s2 n -> n >= 0
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  lemma many_steps_seq:
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    forall sigma1 sigma3:env, pi1 pi3:stack, s1 s2:stmt, n:int.
      many_steps sigma1 pi1 (Sseq s1 s2) sigma3 pi3 Sskip n ->
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      exists sigma2:env, pi2:stack, n1 n2:int.
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        many_steps sigma1 pi1 s1 sigma2 pi2 Sskip n1 /\
        many_steps sigma2 pi2 s2 sigma3 pi3 Sskip n2 /\
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        n = 1 + n1 + n2

 lemma one_step_change_free :
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  forall s s':stmt, sigma sigma':env, pi pi':stack, id:ident, v:value.
    fresh_in_stmt id s ->
    one_step sigma (Cons (id,v) pi) s sigma' pi' s' ->
    one_step sigma pi s sigma' pi' s'
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(** {3 Hoare triples} *)

(** partial correctness *)
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predicate valid_triple (p:fmla) (s:stmt) (q:fmla) =
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    forall sigma:env, pi:stack. eval_fmla sigma pi p ->
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      forall sigma':env, pi':stack, n:int.
        many_steps sigma pi s sigma' pi' Sskip n ->
          eval_fmla sigma' pi' q
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(*** total correctness *)
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predicate total_valid_triple (p:fmla) (s:stmt) (q:fmla) =
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    forall sigma:env, pi:stack. eval_fmla sigma pi p ->
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      exists sigma':env, pi':stack, n:int.
        many_steps sigma pi s sigma' pi' Sskip n /\
        eval_fmla sigma' pi' q
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end


theory TestSemantics

use import ImpExpr

function my_sigma : env = IdMap.const (Vint 0)
constant x : ident
constant y : mident

function my_pi : stack = Cons (x, Vint 42) Nil

goal Test13 :
  eval_term my_sigma my_pi (mk_tvalue (Vint 13)) = Vint 13

goal Test42 :
  eval_term my_sigma my_pi (mk_tvar x) = Vint 42

goal Test0 :
  eval_term my_sigma my_pi (mk_tderef y) = Vint 0

goal Test55 :
  eval_term my_sigma my_pi (mk_tbin (mk_tvar x) Oplus (mk_tvalue (Vint 13))) = Vint 55

goal Ass42 :
  forall sigma':env, pi':stack.
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    one_step my_sigma my_pi (Sassign y (mk_tvalue (Vint 42))) sigma' pi' Sskip ->
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      IdMap.get sigma' y = Vint 42

goal If42 :
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    forall sigma1 sigma2:env, pi1 pi2:stack, s:stmt.
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      one_step my_sigma my_pi
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        (Sif (mk_tbin (mk_tderef y) Ole (mk_tvalue (Vint 10)))
             (Sassign y (mk_tvalue (Vint 13)))
             (Sassign y (mk_tvalue (Vint 42))))
        sigma1 pi1 s ->
      one_step sigma1 pi1 s sigma2 pi2 Sskip ->
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        IdMap.get sigma2 y = Vint 13

end

(** {2 Hoare logic} *)

theory HoareLogic

use import ImpExpr


(** Hoare logic rules (partial correctness) *)

lemma consequence_rule:
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  forall p p' q q':fmla, s:stmt.
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  valid_fmla (Fimplies p' p) ->
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  valid_triple p s q ->
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  valid_fmla (Fimplies q q') ->
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  valid_triple p' s q'
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lemma skip_rule:
  forall q:fmla. valid_triple q Sskip q
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lemma assign_rule:
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  forall p:fmla, x:mident, id:ident, t:term.
  fresh_in_fmla id p ->
  valid_triple (Flet id t (msubst p x id)) (Sassign x t) p
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lemma seq_rule:
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  forall p q r:fmla, s1 s2:stmt.
  valid_triple p s1 r /\ valid_triple r s2 q ->
  valid_triple p (Sseq s1 s2) q
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lemma if_rule:
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  forall t:term, p q:fmla, s1 s2:stmt.
  valid_triple (Fand p (Fterm t)) s1 q /\
  valid_triple (Fand p (Fnot (Fterm t))) s2 q ->
  valid_triple p (Sif t s1 s2) q
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lemma assert_rule:
  forall f p:fmla. valid_fmla (Fimplies p f) ->
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  valid_triple p (Sassert f) p
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lemma assert_rule_ext:
  forall f p:fmla.
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  valid_triple (Fimplies f p) (Sassert f) p
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(*
lemma while_rule:
  forall e:term, inv:fmla, i:expr.
  valid_triple (Fand (Fterm e) inv) i inv ->
  valid_triple inv (Swhile e inv i) (Fand (Fnot (Fterm e)) inv)

lemma while_rule_ext:
  forall e:term, inv inv':fmla, i:expr.
  valid_fmla (Fimplies inv' inv) ->
  valid_triple (Fand (Fterm e) inv') i inv' ->
  valid_triple inv' (Swhile e inv i) (Fand (Fnot (Fterm e)) inv')
*)

(*** frame rule ? *)

end

(** {2 WP calculus} *)

theory WP

use import ImpExpr
use import bool.Bool

use set.Set

(** [assigns sigma W sigma'] is true when the only differences between
    [sigma] and [sigma'] are the value of references in [W] *)

predicate assigns (sigma:env) (a:Set.set mident) (sigma':env) =
  forall i:mident. not (Set.mem i a) ->
    IdMap.get sigma i = IdMap.get sigma' i

lemma assigns_refl:
  forall sigma:env, a:Set.set mident. assigns sigma a sigma

lemma assigns_trans:
  forall sigma1 sigma2 sigma3:env, a:Set.set mident.
    assigns sigma1 a sigma2 /\ assigns sigma2 a sigma3 ->
    assigns sigma1 a sigma3

lemma assigns_union_left:
  forall sigma sigma':env, s1 s2:Set.set mident.
    assigns sigma s1 sigma' -> assigns sigma (Set.union s1 s2) sigma'

lemma assigns_union_right:
  forall sigma sigma':env, s1 s2:Set.set mident.
    assigns sigma s2 sigma' -> assigns sigma (Set.union s1 s2) sigma'

(** [expr_writes e W] is true when the only references modified by [e] are in [W] *)
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predicate stmt_writes (s:stmt) (w:Set.set mident) =
  match s with
  | Sskip | Sassert _ -> true
  | Sassign id _ -> Set.mem id w
  | Sseq s1 s2 -> stmt_writes s1 w /\ stmt_writes s2 w
  | Sif t s1 s2 -> stmt_writes s1 w /\ stmt_writes s2 w
  | Swhile _ _ body -> stmt_writes body w
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  end

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  function fresh_from (f:fmla) (s:stmt) : ident
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  (* Need it for monotonicity*)
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  axiom fresh_from_fmla: forall s:stmt, f:fmla.
     fresh_in_fmla (fresh_from f s) f
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  axiom fresh_from_stmt: forall s:stmt, f:fmla.
     fresh_in_stmt (fresh_from f s) s
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  function abstract_effects (s:stmt) (f:fmla) : fmla
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  axiom abstract_effects_generalize :
     forall sigma:env, pi:stack, s:stmt, f:fmla.
        eval_fmla sigma pi (abstract_effects s f) ->
        eval_fmla sigma pi f

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  axiom abstract_effects_monotonic :
     forall s:stmt, f:fmla.
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        forall sigma:env, pi:stack. eval_fmla sigma pi f ->
        forall sigma:env, pi:stack. eval_fmla sigma pi (abstract_effects s f)
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  function wp (s:stmt) (q:fmla) : fmla =
    match s with
    | Sskip -> q
    | Sassert f ->
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        (* asymmetric and *)
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        Fand f (Fimplies f q)
    | Sseq s1 s2 -> wp s1 (wp s2 q)
    | Sassign x t ->
        let id = fresh_from q s in
        Flet id t (msubst q x id)
    | Sif t s1 s2 ->
        Fand (Fimplies (Fterm t) (wp s1 q))
             (Fimplies (Fnot (Fterm t)) (wp s2 q))
    | Swhile cond inv body ->
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        Fand inv
          (abstract_effects body
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            (Fand
              (Fimplies (Fand (Fterm cond) inv) (wp body inv))
              (Fimplies (Fand (Fnot (Fterm cond)) inv) q)))
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    end

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  axiom abstract_effects_writes :
     forall sigma:env, pi:stack, s:stmt, q:fmla.
        eval_fmla sigma pi (abstract_effects s q) ->
        eval_fmla sigma pi (wp s (abstract_effects s q))


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  (* lemma wp_subst: *)
  (*   forall e:expr, q:fmla, id :mident, id':ident. *)
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  (*   fresh_in_stmt id e -> *)
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  (*     subst (wp e q) id id' = wp e (subst q id id') *)

  lemma monotonicity:
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    forall s:stmt, p q:fmla.
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      valid_fmla (Fimplies p q)
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     ->	valid_fmla (Fimplies (wp s p) (wp s q) )
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  lemma distrib_conj:
    forall s:stmt, sigma:env, pi:stack, p q:fmla.
     (eval_fmla sigma pi (wp s p)) /\
     (eval_fmla sigma pi (wp s q)) ->
     eval_fmla sigma pi (wp s (Fand p q)) 
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  lemma wp_reduction:
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    forall sigma sigma':env, pi pi':stack, s s':stmt.
    one_step sigma pi s sigma' pi' s' ->
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    forall q:fmla.
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      eval_fmla sigma pi (wp s q) ->
      eval_fmla sigma' pi' (wp s' q)
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  lemma progress:
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    forall s:stmt, sigma:env, pi:stack,
      sigmat: type_env, pit: type_stack, q:fmla.
      type_stmt sigmat pit s ->
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(* useful ?
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      type_fmla sigmat pit q ->
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*)
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      eval_fmla sigma pi (wp s q) -> 
      s <> Sskip ->
      exists sigma':env, pi':stack, s':stmt.
      one_step sigma pi s sigma' pi' s'
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end


(***
Local Variables:
compile-command: "why3ide blocking_semantics3.mlw"
End:
*)