private static Optional<String> findCombinedId(ModelElement nodesElement, ModelElement resolvedElement) {
        if (resolvedElement != nodesElement) {
            // Specification for a container cluster referencing nodes in a content cluster
            return containerIdOf(nodesElement);
        }
        // Specification for a content cluster that is referenced by a container cluster
        return containerIdReferencing(nodesElement);
    }
Returns the ID of the cluster referencing this node specification, if any
config-model/src/main/java/com/yahoo/vespa/model/builder/xml/dom/NodesSpecification.java