simp only [Function.comp_def, imCLM_apply, ofReal_im, deriv_const] at H
𝕂 : Type u_1
inst✝ : _root_.RCLike 𝕂
z : ℝ
f : ℝ → ℝ
u : ℂ
hf : HasDerivAt (fun y => ↑(f y)) u z
H : _root_.deriv (⇑imCLM ∘ fun y => ↑(f y)) z = (imCLM.comp (smulRight 1 u)) 1
⊢ u.im = 0
𝕂 : Type u_1
inst✝ : _root_.RCLike 𝕂
z : ℝ
f : ℝ → ℝ
u : ℂ
hf : HasDerivAt (fun y => ↑(f y)) u z
H : 0 = (imCLM.comp (smulRight 1 u)) 1
⊢ u.im = 0