refine sub_ne_zero.mpr (mt (congr_arg Complex.abs) (ne_comm.mp (ne_of_lt ?_)))
z u z₀ : ℂ
U : Set ℂ
inst✝ : good_domain U
hu : ‖u‖ < 1
hz : ‖z‖ < 1
⊢ 1 - z * (starRingEnd ℂ) u ≠ 0
z u z₀ : ℂ
U : Set ℂ
inst✝ : good_domain U
hu : ‖u‖ < 1
hz : ‖z‖ < 1
⊢ Complex.abs (z * (starRingEnd ℂ) u) < Complex.abs 1