code: function(node, levelBegin, levelEnd, action, flags) {
          var d = node._depth, filter = this.filter(flags), that = this;
          levelEnd = levelEnd === false? Number.MAX_VALUE -d : levelEnd;
          (function loopLevel(node, levelBegin, levelEnd) {
              var d = node._depth;
              if(d >= levelBegin && d <= levelEnd && filter(node)) action(node, d);
              if(d < levelEnd) {
                  that.eachAdjacency(node, function(adj) {
                      var n = adj.nodeTo;
                      if(n._depth > d) loopLevel(n, levelBegin, levelEnd);
                  });
              }
          })(node, levelBegin + d, levelEnd + d);      
      }
docs: /*
  Method: eachLevel
  
  Iterates over a node's subgraph applying *action* to the nodes of relative depth between *levelBegin* and *levelEnd*.
  
  Also implemented by:
  
  <Graph.Node>.
  
  Parameters:
  
  node - (object) A <Graph.Node>.
  levelBegin - (number) A relative level value.
  levelEnd - (number) A relative level value.
  action - (function) A callback function having a <Graph.Node> as first formal parameter.