. exfalso
  apply of_maxLiteral_eq_none g h2 a h
case h_1
a : Nat
g : CNF Nat
h : mem a g
x✝ : Option Nat
h2 : g.maxLiteral = none
⊢ a < 0

case h_2
a : Nat
g : CNF Nat
h : mem a g
x✝ : Option Nat
n✝ : Nat
h2 : g.maxLiteral = some n✝
⊢ a < n✝ + 1
case h_2
a : Nat
g : CNF Nat
h : mem a g
x✝ : Option Nat
n✝ : Nat
h2 : g.maxLiteral = some n✝
⊢ a < n✝ + 1