private Set<Version> keepThoseUsedOn(AllocatedHosts hosts, Set<Version> versions) {
        return versions.stream().filter(version -> isUsedOn(hosts, version)).collect(Collectors.toSet());
    }
Returns the subset of the given versions which are in use on these hosts
configserver/src/main/java/com/yahoo/vespa/config/server/modelfactory/ModelsBuilder.java