private Class<? extends Identifiable> get(int id) {
            var pair = typeMap.get(id);
            return (pair != null) ? pair.getFirst() : null;
        }
Returns the class registered for the given identifier.

@param id The identifer whose class to return.
@return The class specification, may be null.
vespajlib/src/main/java/com/yahoo/vespa/objects/Identifiable.java