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) (bigCon 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) (bigCon X⋁discon XS)