(** As mentioned previously, two locations can be different (in the sense
  of the [<>] mathematical disequality), yet denote
  overlapping memory chunks within the activation record.
  Given two locations, three cases are possible:
- They are equal (in the sense of the [=] equality)
- They are different and non-overlapping.
- They are different but overlapping.

  The second case (different and non-overlapping) is characterized
  by the following [Loc.diff] predicate.
*)
  Definition diff (l1 l2: loc) : Prop :=
    match l1, l2 with
    | R r1, R r2 =>
        r1 <> r2
    | S s1 d1 t1, S s2 d2 t2 =>
        s1 <> s2 \/ d1 + typesize t1 <= d2 \/ d2 + typesize t2 <= d1
    | _, _ =>
        True
    end.

Lemma diff_sym: forall l1 l2, diff l1 l2 -> diff l2 l1.