protected Source buildChain(DeployState deployState, TreeConfigProducer<AnyConfigProducer> ancestor, Element producerSpec, ChainSpecification specWithoutInnerComponents) {
        Source.GroupOption groupOption =
                XmlHelper.isReference(producerSpec) ?
                        Source.GroupOption.participant :
                        Source.GroupOption.leader;

        return new Source(specWithoutInnerComponents, readFederationOptions(producerSpec), groupOption);
    }
Builds a source from xml.
@author Tony Vaagenes
config-model/src/main/java/com/yahoo/vespa/model/builder/xml/dom/chains/search/DomSourceBuilder.java