public void addDescendant(String configId, AnyConfigProducer descendant) {
        if (id2producer.containsKey(configId)) {
            throw new RuntimeException
                    ("Config ID '" + configId + "' cannot be reserved by an instance of class '" +
                            descendant.getClass().getName() + "' since it is already used by an instance of class '" +
                            id2producer.get(configId).getClass().getName() + "'");
        }
        id2producer.put(configId, descendant);
    }
Use for testing. Use as parent for the config producer(s) you want to test, to test
only a subtree of the producers.

@author gjoranv
config-model/src/main/java/com/yahoo/config/model/test/MockRoot.java