constructor
x : ℕ
ρ φ : Formula
F : List Formula
IH : x ∉ voc F → List.map (repl_in_F x ρ) F = F
nonOcc : x ∉ voc (φ :: F)
⊢ repl_in_F x ρ φ = φ ∧ List.map (repl_in_F x ρ) F = F
case left
x : ℕ
ρ φ : Formula
F : List Formula
IH : x ∉ voc F → List.map (repl_in_F x ρ) F = F
nonOcc : x ∉ voc (φ :: F)
⊢ repl_in_F x ρ φ = φ

case right
x : ℕ
ρ φ : Formula
F : List Formula
IH : x ∉ voc F → List.map (repl_in_F x ρ) F = F
nonOcc : x ∉ voc (φ :: F)
⊢ List.map (repl_in_F x ρ) F = F