open Or in intro
| ⟨inl hs, _⟩ => left; exact hs
| ⟨_, inl hs⟩ => left; exact hs
| ⟨inr ht, inr hu⟩ => right; constructor; exact ht; exact hu
case a.mpr
α : Sort u_1
β : ?m.20770
γ : ?m.20773
s t u : Set α
x : α
⊢ x ∈ (s.union t).inter (s.union u) → x ∈ s.union (t.inter u)
no goals