code: public GroupZipper down() {
          if (!canDown()) {
              throw new NoSuchElementException("Could not move down because this group does not have any children");
          }
  
          parent = new GroupZipper(parent, node, index);
          index = 0;
          node = node.getGroups().get(0);
  
          return this;
      }
docs: Navigates down the tree to the first child node of the current node.
  <p>
  If the current node has no childs an exception will be thrown.
  
  @return a new groupzipper which points to the first child node of the
  current node
  @throws NoSuchElementException
  if the current node has no child nodes