code: function(fn) {
          var values = [];
          for (var i in this._nodes) {
              if (this._nodes.hasOwnProperty(i)) {
                  var n = this._nodes[i];
                  if (fn(n)) {
                      values.push(n);
                  }
              }
          }
          return (values.length) ? values : null;
      }
docs: Returns a collection of nodes that have passed the test function
  passed as its only argument.
  The function will receive a reference to each node to be tested.
  @method getNodesBy
  @param {function} a boolean function that receives a Node instance and returns true to add the node to the results list
  @return {Array} the matching collection of nodes, null if no match