protected void onSerialize(Serializer buf) {
        // empty
    }
Serializes the content of this class into the given
buffer. This method must be implemented by all subclasses that
have content. If the subclass has no other content than the
semantics of its class type, this method does not need to be
overloaded.

@param buf The buffer to serialize to.
vespajlib/src/main/java/com/yahoo/vespa/objects/Identifiable.java