public ClusterSpec clusterSpec() {
        ensureSingleCluster();
        if (isEmpty()) throw new IllegalStateException("No nodes");
        return first().get().allocation().get().membership().cluster().with(Optional.empty());
    }
Returns the cluster spec of the nodes in this, without any group designation

@throws IllegalStateException if there are no nodes in this list, or they do not all belong to the same cluster
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/NodeList.java