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

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