private void doElementSpecificProcessingOnOverride(List<Element> elements) {
        // if node capacity is specified explicitly for some combination we should require that capacity
        elements.forEach(element -> {
            if (element.getTagName().equals("nodes"))
                if (!hasChildWithTagName(element, "node")) // specifies capacity, not a list of nodes
                    element.setAttribute("required", "true");
        });
    }
Called on each element which is selected by matching some override condition
config-application-package/src/main/java/com/yahoo/config/application/OverrideProcessor.java