docs: def get_instances(self, state_filter=None):
      """
      Get all the instances filtered by state.
      
      @param state_filter: the state that the instance should be in
        (e.g. "running"), or None for all states
      """
  
      instances = []
      for instance in self._get_instances(self._get_cluster_group_name(), 
                                          state_filter):
        instances.append(Instance(instance.id, instance.dns_name, 
                                  instance.private_dns_name, 
                                  instance.private_ip_address))
      return instances