code: public GroupZipper up() {
          if (!canUp()) {
              throw new NoSuchElementException("Could not move up because this group does not have a parent");
          }
  
          this.index = parent.index;
          this.node = parent.node;
          this.parent = parent.parent;
  
          return this;
      }
docs: Navigates up the tree to the parent node of the current node.
  <p>
  If the current node has no parent an exception will be thrown.
  
  @return a new groupzipper which points to the parent node of the current
  node
  @throws NoSuchElementException
  if the current node has no parent node