tros.
  rewrite <- (Int.repr_unsigned i), <- (Int.repr_unsigned j).
  rewrite H.
  reflexivity.
Qed.

Ltac solve_mod_eq :=
  unfold Int.add, Int.mul;
  repeat rewrite Int.unsigned_repr_eq;
  repeat
  (repeat rewrite Zmod_mod;

Hint Rewrite wand_sepcon wand_sepcon' : norm. Lemma extract_nth_exists_in_SEP: forall n P Q (R: list mpred)