by_cases hpol : polarity l₁ = polarity l₂
case mp
l₁ l₂ : ILit
hlit : toVar l₁ = toVar l₂
⊢ l₁ = l₂ ∨ negate l₁ = l₂
case pos
l₁ l₂ : ILit
hlit : toVar l₁ = toVar l₂
hpol : polarity l₁ = polarity l₂
⊢ l₁ = l₂ ∨ negate l₁ = l₂

case neg
l₁ l₂ : ILit
hlit : toVar l₁ = toVar l₂
hpol : ¬polarity l₁ = polarity l₂
⊢ l₁ = l₂ ∨ negate l₁ = l₂