(* The next line is "boilerplate", always required after importing an AST. *)
Instance CompSpecs : compspecs. make_compspecs prog. Defined.
Definition Vprog : varspecs.  mk_varspecs prog. Defined.
(* Some definitions relating to the functional spec of this particular program.  *)
Definition sum_Z : list Z -> Z := fold_right Z.add 0.
(* Beginning of the API spec for the sumarray.c program *)
Definition sumarray_spec :=
DECLARE _sumarray
WITH a: val, sh : share, contents : list Z, size: Z
PRE [ _a OF (tptr tuint), _n OF tint ]
PROP  (readable_share sh; 0 <= size <= Int.max_signed)
LOCAL (temp _a a; temp _n (Vint (Int.repr size)))
SEP   (data_at sh (tarray tuint size) (map Vint (map Int.repr contents)) a)
POST [ tuint ]
PROP () LOCAL(temp ret_temp  (Vint (Int.repr (sum_Z contents))))
SEP (data_at sh (tarray tuint size) (map Vint (map Int.repr contents)) a).
(* The precondition of "int main(void){}" always looks like this. *)
Definition main_spec :=
DECLARE _main
WITH gv: globals
PRE  [] main_pre prog tt nil gv
POST [ tint ]
PROP()
LOCAL (temp ret_temp (Vint (Int.repr (3+4))))
SEP(TT).
(* Packaging the API spec all together. *)
Definition Gprog : funspecs :=
ltac:(with_library prog [sumarray_spec; main_spec]).
(* Contents of the extern global initialized array "_four" *)
Definition four_contents := [1; 2; 3; 4].
Existing Instance NullExtension.Espec.
(**  Here begins an alternate proof of the "for" loop. Instead of using forward_for_simple_bound, we use the general-case loop tactic, forward_for.  To understand this verification, let's take the program,  int sumarray(int a[], int n) { int i,s,x; s=0; for (i=0; i<n; i++) { x = a[i]; s += x; } return s; }  and annotate with assertions:   int sumarray(int a[], int n) { int i,s,x; s=0; for (i=0; i<n; i++) { assert (sumarray_Inv(i)); x = a[i]; s += x; assert (sumarray_PostBody(i)); } return s; }  The assertions are defined in these definitions: *)
Definition sumarray_Inv a sh contents size i :=
PROP  (0 <= i <= size)
LOCAL (temp _a a;
temp _i (Vint (Int.repr i));
temp _n (Vint (Int.repr size));
temp _s (Vint (Int.repr (sum_Z (sublist 0 i contents)))))
SEP   (data_at sh (tarray tuint size) (map Vint (map Int.repr contents)) a).
Definition sumarray_PostBody a sh contents size i :=
PROP  (0 <= i < size)
LOCAL (temp _a a;
temp _i (Vint (Int.repr i));
temp _n (Vint (Int.repr size));
temp _s (Vint (Int.repr (sum_Z (sublist 0 (i+1) contents)))))
SEP   (data_at sh (tarray tuint size) (map Vint (map Int.repr contents)) a).
(* . . . and now you can see how these assertions are used in the proof, using the semax_loop rule. *)

Lemma body_sumarray_alt: semax_body Vprog Gprog f_sumarray sumarray_spec.