A Strategy for Defining Order Relations - http://www.drdobbs.com/cpp/a-strategy-for-defining-order-relations/240147625?cid=DDJ_nl_upd_2013-02-06_h&elq=fa440bebd1594befbf7ba36a8e9de0e6&token=47926b093170e9c983515df7c338da1d