docs: def user(self, username=None, pk=None, **kwargs):
          """
          User of KE-chain.
  
          Provides single user of :class:`User` of KE-chain. You can filter on username or id or an advanced filter.
  
          :param username: (optional) username to filter
          :type username: basestring or None
          :param pk: (optional) id of the user to filter
          :type pk: basestring or None
          :param kwargs: Additional filtering keyword=value arguments
          :type kwargs: dict or None
          :return: List of :class:`User`
          :raises NotFoundError: when a user could not be found
          :raises MultipleFoundError: when more than a single user can be found
          """
          _users = self.users(username=username, pk=pk, **kwargs)
  
          if len(_users) == 0:
              raise NotFoundError("No user criteria matches")
          if len(_users) != 1:
              raise MultipleFoundError("Multiple users fit criteria")
  
          return _users[0]