code: public PayloadType<ConstraintType<T>> getOrCreatePayload()
     {
        Node node = childNode.getOrCreate("payload");
        PayloadType<ConstraintType<T>> payload = new PayloadTypeImpl<ConstraintType<T>>(this, "payload", childNode, node);
        return payload;
     }
docs: If not already created, a new <code>payload</code> element with the given value will be created.
  Otherwise, the existing <code>payload</code> element will be returned.
  @return  a new or existing instance of <code>PayloadType<ConstraintType<T>></code>