1 function d3_transition_each(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) callback.call(node = node.node, node.__data__, i, j);

     6     }

     7   }

     8   return this;

     9 }