private static Optional<Element> findChildById(Element parent, String id) {
        for (Element child : XML.getChildren(parent))
            if (id.equals(child.getAttribute("id"))) return Optional.of(child);
        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