export default function(callback, that) {
  var node = this, nodes = [node], children, i, index = -1;
  while (node = nodes.pop()) {
    callback.call(that, node, ++index, this);
    if (children = node.children) {
      for (i = children.length - 1; i >= 0; --i) {
        nodes.push(children[i]);
      }
    }
  }
  return this;
}