private CommentedSearchChain commentedSearchChain(SingleTarget singleTarget, SearchChainRegistry registry) {
        return new CommentedSearchChain("If source refs contains '" + singleTarget.getId() + "'.",
                                        registry.getChain(singleTarget.getId()));
    }
The name of the query property containing the source name added to the query to each source by this
container-search/src/main/java/com/yahoo/search/federation/FederationSearcher.java