public void validate(Deserializer<?> deserializer) {
        rules.stream()
             .flatMap(rule -> rule.getValueToApply().map(Stream::of).orElse(null))
             .forEach(deserializer::deserialize);

    }
E.g. verify all RawFlag can be deserialized.
flags/src/main/java/com/yahoo/vespa/flags/json/FlagData.java