* R*R)} of the population standard deviation of {@code y}, where {@code R} is the Pearson's
   * correlation coefficient (as given by {@link #pearsonsCorrelationCoefficient()}).
   *
   * <p>The corresponding root-mean-square error in {@code x} as a function of {@code y} is a
   * fraction {@code sqrt(1/(R*R) - 1)} of the population standard deviation of {@code x}. This fit

  // 4.1 Validate.

  // Must be NFC.
  if (decoded != normalizeNfc(decoded)) return null

  // TODO: Must not begin with a combining mark.
  // TODO: Each character must be 'valid' or 'deviation'. Not mapped.
  // TODO: CheckJoiners from IDNA 2008
  // TODO: CheckBidi from IDNA 2008, RFC 5893, Section 2.

  return Punycode.encode(decoded)

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00DD          ; mapped                 ; 00FD          # 1.1  LATIN CAPITAL LETTER Y WITH ACUTE
00DE          ; mapped                 ; 00FE          # 1.1  LATIN CAPITAL LETTER THORN
00DF          ; deviation              ; 0073 0073     # 1.1  LATIN SMALL LETTER SHARP S
00E0..00F6    ; valid                                  # 1.1  LATIN SMALL LETTER A WITH GRAVE..LATIN SMALL LETTER O WITH DIAERESIS

    - `apiserver_flowcontrol_demand_seats_average`: Time-weighted average, over last adjustment period, of demand_seats
    - `apiserver_flowcontrol_demand_seats_stdev`: Time-weighted standard deviation, over last adjustment period, of demand_seats
    - `apiserver_flowcontrol_demand_seats_smoothed`: Smoothed seat demands
    - `apiserver_flowcontrol_target_seats`: Seat allocation targets