Module Facts := FSetEqProperties.EqProperties(PositiveSet).
Definition max_element (vars : t)  :=
fold Pos.max vars xH.
Definition is_subset (v1 v2 : t) :=
forall x, mem x v1 = true -> mem x v2 = true.

Lemma is_subset_union_l : forall v1 v2,
is_subset v1 (union v1 v2).