simp only [Formula.boxes, repl_in_F, Formula.box.injEq]
x : ℕ
ψ : Formula
α : Program
δ : List Program
IH : x ∉ voc δ → repl_in_F x ψ (⌈⌈δ⌉⌉~·x) = ⌈⌈δ⌉⌉~ψ
nonOcc : x ∉ voc (α :: δ)
⊢ repl_in_F x ψ (⌈⌈α :: δ⌉⌉~·x) = ⌈⌈α :: δ⌉⌉~ψ
x : ℕ
ψ : Formula
α : Program
δ : List Program
IH : x ∉ voc δ → repl_in_F x ψ (⌈⌈δ⌉⌉~·x) = ⌈⌈δ⌉⌉~ψ
nonOcc : x ∉ voc (α :: δ)
⊢ repl_in_P x ψ α = α ∧ repl_in_F x ψ (⌈⌈δ⌉⌉~·x) = ⌈⌈δ⌉⌉~ψ