d3_selection_enterPrototype.select = function(selector) {
  var subgroups = [],
      subgroup,
      subnode,
      upgroup,
      group,
      node;

  for (var j = -1, m = this.length; ++j < m;) {
    upgroup = (group = this[j]).update;
    subgroups.push(subgroup = []);
    subgroup.parentNode = group.parentNode;
    for (var i = -1, n = group.length; ++i < n;) {
      if (node = group[i]) {
        subgroup.push(upgroup[i] = subnode = selector.call(group.parentNode, node.__data__, i));
        subnode.__data__ = node.__data__;
      } else {
        subgroup.push(null);
      }
    }
  }

  return d3_selection(subgroups);
};
