1 d3_selection_enterPrototype.select = function(selector) {

     2   var subgroups = [],

     3       subgroup,

     4       subnode,

     5       upgroup,

     6       group,

     7       node;

     8 

     9   for (var j = -1, m = this.length; ++j < m;) {

    10     upgroup = (group = this[j]).update;

    11     subgroups.push(subgroup = []);

    12     subgroup.parentNode = group.parentNode;

    13     for (var i = -1, n = group.length; ++i < n;) {

    14       if (node = group[i]) {

    15         subgroup.push(upgroup[i] = subnode = selector.call(group.parentNode, node.__data__, i));

    16         subnode.__data__ = node.__data__;

    17       } else {

    18         subgroup.push(null);

    19       }

    20     }

    21   }

    22 

    23   return d3_selection(subgroups);

    24 };