Definition memory_block (sh: share) (n: Z) (v: val) : mpred :=
match v with
| Vptr b ofs => (!!(Ptrofs.unsigned ofs + n < Ptrofs.modulus)) && memory_block' sh (nat_of_Z n) b (Ptrofs.unsigned ofs)
| _ => FF
end.

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.