. rintro (hp | hq);
  . apply membership_iff.mpr;
    exact disj₁'! (membership_iff.mp hp);
  . apply membership_iff.mpr;
    exact disj₂'! (membership_iff.mp hq);
case mpr
α : Type u_1
inst✝² : DecidableEq α
inst✝¹ : Inhabited α
L : DeductionParameter α
inst✝ : L.Normal
Ω Ω₁ Ω₂ : MCT L
p q : Formula α
⊢ p ∈ Ω.theory ∨ q ∈ Ω.theory → p ⋎ q ∈ Ω.theory
no goals