∀ (c : UInt8) (_h : c.IsUTF8FirstByte) (motive : ℕ → Prop),
  (c &&& 128 = 0 → motive 1) →
    (¬c &&& 128 = 0 → c &&& 224 = 192 → motive 2) →
      (¬c &&& 128 = 0 → ¬c &&& 224 = 192 → c &&& 240 = 224 → motive 3) →
        (¬c &&& 128 = 0 → ¬c &&& 224 = 192 → ¬c &&& 240 = 224 → motive 4) → motive (c.utf8ByteSize _h)