private static Optional<String> containerIdReferencing(ModelElement nodesElement) {
        var element = nodesElement.getXml();
        var services = findParentByTag("services", element);
        if (services.isEmpty()) return Optional.empty();

        var content = findParentByTag("content", element);
        if (content.isEmpty()) return Optional.empty();
        var contentClusterId = content.get().getAttribute("id");
        if (contentClusterId.isEmpty()) return Optional.empty();
        for (var rootChild : XML.getChildren(services.get())) {
            if ( ! ContainerModelBuilder.isContainerTag(rootChild)) continue;
            var nodes = XML.getChild(rootChild, "nodes");
            if (nodes == null) continue;
            if (!contentClusterId.equals(nodes.getAttribute("of"))) continue;
            return Optional.of(rootChild.getAttribute("id"));
        }
        return Optional.empty();
    }
Returns the ID of the container element referencing nodesElement, if any
config-model/src/main/java/com/yahoo/vespa/model/builder/xml/dom/NodesSpecification.java