simp only [deriv_add_const', _root_.deriv_ofReal, mul_one] at this
case h.refine_2
t : ℝ
f : ℂ → ℂ
diff : ∀ (σ : ℝ), DifferentiableAt ℂ f (↑σ + ↑t * I)
σ : ℝ
this : deriv (f ∘ fun σ => ↑σ + ↑t * I) σ = deriv f ((fun σ => ↑σ + ↑t * I) σ) * deriv (fun σ => ↑σ + ↑t * I) σ
⊢ deriv (fun {σ₂} => f (↑σ₂ + ↑t * I)) σ = deriv f (↑σ + ↑t * I)
case h.refine_2
t : ℝ
f : ℂ → ℂ
diff : ∀ (σ : ℝ), DifferentiableAt ℂ f (↑σ + ↑t * I)
σ : ℝ
this : deriv (f ∘ fun σ => ↑σ + ↑t * I) σ = deriv f (↑σ + ↑t * I)
⊢ deriv (fun {σ₂} => f (↑σ₂ + ↑t * I)) σ = deriv f (↑σ + ↑t * I)