antisymmetry using (.⊆.)
α : Sort u_1
s t u : Set α
hs : Set.univ.Subset s
⊢ s = Set.univ
case a
α : Sort u_1
s t u : Set α
hs : Set.univ.Subset s
⊢ s.Subset Set.univ

case a
α : Sort u_1
s t u : Set α
hs : Set.univ.Subset s
⊢ Set.univ.Subset s