docs: def validator(node, rrn):
          '''
          Colander validator that checks whether a given value is a valid
          Belgian national ID number .
          :raises colander.Invalid: If the value is no valid Belgian national ID number.
          '''
  
          def _valid(_rrn):
              x = 97 - (int(_rrn[:-2]) - (int(_rrn[:-2]) // 97) * 97)
              return int(_rrn[-2:]) == x
  
          if len(rrn) != 11:
              raise colander.Invalid(
                  node,
                  'Een rijksregisternummer moet 11 cijfers lang zijn.'
              )
          elif not _valid(rrn):
              valid_rrn = False
              if rrn[:1] == '0' or rrn[:1] == '1':
                  rrn = '2' + rrn
                  valid_rrn = _valid(rrn)
              if not valid_rrn:
                  raise colander.Invalid(
                      node,
                      'Dit is geen correct rijksregisternummer.'
                  )