func (c Clusters) Primaries(excluded ...Cluster) Clusters {
	return c.filterClusters(func(cc Cluster) bool {
		return cc.IsPrimary()
	}, exclude(excluded...))
}

// Exclude returns all clusters not given as input.
func (c Clusters) Exclude(excluded ...Cluster) Clusters {
	return c.filterClusters(func(cc Cluster) bool {
		return true
	}, exclude(excluded...))
}