∀ (c : UInt8),
  Std.Http.Internal.Char.isHexDigitByte c =
    (decide (c ≥ '0'.toUInt8) && decide (c ≤ '9'.toUInt8) || decide (c ≥ 'a'.toUInt8) && decide (c ≤ 'f'.toUInt8) ||
      decide (c ≥ 'A'.toUInt8) && decide (c ≤ 'F'.toUInt8))