docs: def istring(self, in_string=''):
          """Return a string that uses this server's IRC casemapping.
  
          This string's equality with other strings, ``lower()``, and ``upper()`` takes this
          server's casemapping into account. This should be used for things such as nicks and
          channel names, where comparing strings using the correct casemapping can be very
          important.
          """
          new_string = IString(in_string)
          new_string.set_std(self.features.get('casemapping'))
          if not self._casemap_set:
              self._imaps.append(new_string)
          return new_string