simp [voc,vocabOfProgram,vocabOfListFormula] at *
case left.a
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)
⊢ x ∉ voc φ
case left.a
x : ℕ
ρ φ : Formula
F : List Formula
IH : (∀ x_1 ∈ F, x ∉ vocabOfFormula x_1) → List.map (repl_in_F x ρ) F = F
nonOcc : x ∉ vocabOfFormula φ ∧ ∀ x_1 ∈ F, x ∉ vocabOfFormula x_1
⊢ x ∉ vocabOfFormula φ