Section Subsume.
Variable A : Type.
Variable sub : A -> A -> Prop.
Variable sub_reflexive:
forall a,
sub a a.
Variable sub_transitive:
forall a1 a2 a3,
sub a1 a2 ->
sub a2 a3 ->
sub a1 a3.
(* Subsumption is first extended to options. *)
Definition osub (o1 o2 : option A) :=
forall a2,
o2 = Some a2 ->
exists a1,
o1 = Some a1 /\ sub a1 a2.

Lemma osub_Some_inversion:
forall o1 a2,
osub o1 (Some a2) ->
exists a1,
o1 = Some a1 /\ sub a1 a2.