Fixpoint memory_block' (sh: share) (n: nat) (b: block) (i: Z) : mpred :=
  match n with
  | O => emp
  | S n' => mapsto_ sh (Tint I8 Unsigned noattr) (Vptr b (Ptrofs.repr i))
         * memory_block' sh n' b (i+1)
 end.Definition memory_block'_alt (sh: share) (n: nat) (b: block) (ofs: Z) : mpred :=
 if readable_share_dec sh 
 then VALspec_range (Z_of_nat n) sh (b, ofs)
 else nonlock_permission_bytes sh (b,ofs) (Z.of_nat n).

Lemma Nat2Z_add_le: forall n i, Ptrofs.unsigned i + n <= Ptrofs.modulus -> Z.of_nat (nat_of_Z n) + Ptrofs.unsigned i <= Ptrofs.modulus.