var xpath   = {
  'node' : {     // any node i.e. thread or subthread
    'desc' : 'Thread or sub-thread',
      'p'  : {   // key to enter/browse 'Replies' collection
        'desc'   : 'Replies',
        xpath    : function () { return "//tr[@class='reply']" },
        'l' : {  
          'desc'   : 'go to node title links',
          xpath    : function (i) {
                         return <XPath expr selecting links based
                                 on 'Replies' collection index>
          },
        },
      'b' : {
        'desc'   : 'go to node body links',
        xpath    : function (i) { ... },
            },
        },
    }
};