/* I'm not thrilled with this, but it should be enough to demonstrate ... something ... */
  function Breadcrumb(el) {
    this.path = el.parentElement ? new Breadcrumb(el.parentElement) : null;
    this.left = el.previousElementSibling;
    this.right = el.nextElementSibling;
  }

  function Zipper(tree, path) {
    this.tree = tree;
    this.crumb = new Breadcrumb(tree);
  }

  function hierarchyPanic(msg) {
    throw new DOMError("HierarchyRequestError", msg);
  }

  Zipper.prototype.goLeft = function () {
    if (!this.crumb.path) hierarchyPanic("cannot go left of top");
    if (!this.crumb.left) hierarchyPanic("cannot go left of first");
    return new Zipper(this.crumb.left);
  }

  Zipper.prototype.goRight = function () {
    if (!this.crumb.path) hierarchyPanic("cannot go right of top");
    if (!this.crumb.right) hierarchyPanic("cannot go left of last");
    return new Zipper(this.crumb.right);
  }

  Zipper.prototype.goUp = function () {
    if (!this.crumb.path) hierarchyPanic("cannot go up of top");
    return new Zipper(this.tree.parentElement);
  }

  Zipper.prototype.goDown = function () {
    var child = this.tree.firstElementChild
    if (!child) hierarchyPanic("cannot go down from leaf");
    return new Zipper(child);
  }

  Zipper.prototype.change = function (el) {
    this.tree.parentElement.replaceChild(el, this.tree);
    return new Zipper(el);
  }

  Zipper.prototype.insertRight = function (el) {
    this.tree.parentElement.insertBefore(el, this.crumb.right);
    return new Zipper(this.tree);
  }

  Zipper.prototype.insertLeft = function (el) {
    this.tree.parentElement.insertBefore(el, this.tree);
    return new Zipper(this.tree);
  }
  
  Zipper.prototype.insertDown = function (el) {
    this.tree.appendChild(el);
    return new Zipper(el);
  }