Definition is_loc (a: expr) : option (block * ptrofs * type) :=
  match a with
  | Eloc b ofs ty => Some(b, ofs, ty)
  | _ => None
  end.

(* Specifications: *)
Lemma is_loc_inv: forall a b ofs ty, is_loc a = Some(b, ofs, ty) -> a = Eloc b ofs ty.