intro XS W M w
X : List Formula
⊢ ∀ (XS : List (List Formula)) (W : Type) (M : KripkeModel W) (w : W),
    evaluate M w (discon (X :: XS)) ↔ evaluate M w (Con X⋁discon XS)
X : List Formula
XS : List (List Formula)
W : Type
M : KripkeModel W
w : W
⊢ evaluate M w (discon (X :: XS)) ↔ evaluate M w (Con X⋁discon XS)