intro rhs
case mpr
XS YS : List (List Formula)
W : Type
M : KripkeModel W
w : W
⊢ ((¬∃ Y ∈ XS, ∀ f ∈ Y, Evaluate (M, w) f) → ∃ Y ∈ YS, ∀ f ∈ Y, Evaluate (M, w) f) →
    ∃ Y, (Y ∈ XS ∨ Y ∈ YS) ∧ ∀ f ∈ Y, Evaluate (M, w) f
case mpr
XS YS : List (List Formula)
W : Type
M : KripkeModel W
w : W
rhs : (¬∃ Y ∈ XS, ∀ f ∈ Y, Evaluate (M, w) f) → ∃ Y ∈ YS, ∀ f ∈ Y, Evaluate (M, w) f
⊢ ∃ Y, (Y ∈ XS ∨ Y ∈ YS) ∧ ∀ f ∈ Y, Evaluate (M, w) f