1 d3_selectionPrototype.each = function(callback) {

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

     3     for (var group = this[j], i = -1, n = group.length; ++i < n;) {

     4       var node = group[i];

     5       if (node) callback.call(node, node.__data__, i, j);

     6     }

     7   }

     8   return this;

     9 };