1 d3_selectionPrototype.node = function(callback) {

     2   for (var j = 0, m = this.length; j < m; j++) {

     3     for (var group = this[j], i = 0, n = group.length; i < n; i++) {

     4       var node = group[i];

     5       if (node) return node;

     6     }

     7   }

     8   return null;

     9 };