to_set_union :
  ∀ {A : Type} [deceq : decidable_eq A] (s t : finset A),
    @eq (set A) (@ts A (@union A deceq s t)) (@set.union A (@ts A s) (@ts A t))
to_set_empty : ∀ {A : Type}, @eq (set A) (@ts A (@empty A)) (@set.empty A)