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