docs: def in6_mactoifaceid(mac, ulbit=None):
      """
      Compute the interface ID in modified EUI-64 format associated
      to the Ethernet address provided as input.
      value taken by U/L bit in the interface identifier is basically
      the reversed value of that in given MAC address it can be forced
      to a specific value by using optional 'ulbit' parameter.
      """
      if len(mac) != 17:
          return None
      m = "".join(mac.split(':'))
      if len(m) != 12:
          return None
      first = int(m[0:2], 16)
      if ulbit is None or not (ulbit == 0 or ulbit == 1):
          ulbit = [1, '-', 0][first & 0x02]
      ulbit *= 2
      first = "%.02x" % ((first & 0xFD) | ulbit)
      eui64 = first + m[2:4] + ":" + m[4:6] + "FF:FE" + m[6:8] + ":" + m[8:12]
      return eui64.upper()