∀ {E : Type u} [inst : NormedAddCommGroup E] [inst_1 : NormedSpace ℂ E] [CompleteSpace E] {R : ℝ} {c w : ℂ} {f : ℂ → E},
  DifferentiableOn ℂ f (Metric.closedBall c R) →
    w ∈ Metric.ball c R → ∮ (z : ℂ) in C(c, R), (z - w)⁻¹ • f z = (2 * ↑Real.pi * Complex.I) • f w