private static void addIndexingChain(ContainerCluster<?> containerCluster) {
        DocprocChain chainAlreadyPresent = containerCluster.getDocprocChains().allChains().
                getComponent(new ComponentId(IndexingDocprocChain.NAME));
        if (chainAlreadyPresent != null) {
            if (chainAlreadyPresent instanceof IndexingDocprocChain) return;
            throw new IllegalArgumentException("A docproc chain may not have the ID '" +
                                               IndexingDocprocChain.NAME + ", since this is reserved by Vespa. Please use a different ID.");
        }

        containerCluster.getDocprocChains().add(new IndexingDocprocChain());
    }
Returns indexing cluster implicitly created by this,
or empty if an explicit cluster is used (or if called before the build phase)
config-model/src/main/java/com/yahoo/vespa/model/content/Content.java