code: func (gce *Cloud) ListInstancesInInstanceGroup(name string, zone string, state string) (*compute.InstanceGroupsListInstances, error) {
  	// TODO: use PageToken to list all not just the first 500
  	return gce.service.InstanceGroups.ListInstances(
  		gce.projectID, zone, name,
  		&compute.InstanceGroupsListInstancesRequest{InstanceState: state}).Do()
  }
docs: // ListInstancesInInstanceGroup lists all the instances in a given instance group and state.