Definition all_equiv_under (l1 l2 : list Prop) :=
forall x y z, In x l1 -> In y l2 -> In z l2 -> (x -> (y <-> z)).

Lemma all_equiv2_impl__stronger : forall l1 l2 x y,
all_equiv l1 -> all_equiv l2 -> In x l1 -> In y l2 -> (x -> y) -> stronger l1 l2.