private ComponentsBuilder<COMPONENT> readOuterComponents(DeployState deployState, TreeConfigProducer<AnyConfigProducer> ancestor, List<Element> chainsElems) {
        return new ComponentsBuilder<>(deployState, ancestor, allowedComponentTypes, chainsElems, null);
    }
NOTE: This class _must_ be abstract, due to calling subclass method in ctor.
@author Tony Vaagenes
@author gjoranv
config-model/src/main/java/com/yahoo/vespa/model/builder/xml/dom/chains/DomChainsBuilder.java