private void addIncludes(Element parentElement) {
        List<Element> includes = XML.getChildren(parentElement, IncludeDirs.INCLUDE);
        if (includes.isEmpty()) {
            return;
        }
        if (app == null) {
            throw new IllegalArgumentException("Element <include> given in XML config, but no application package given.");
        }
        for (Element include : includes) {
            addInclude(parentElement, include);
        }
    }
Allocate a container cluster without a nodes tag
config-model/src/main/java/com/yahoo/vespa/model/container/xml/ContainerModelBuilder.java