code: public GroupZipper replace(Group group) {
          if (parent == null) {
              node = group;
          } else {
              parent.getNode().getGroups().set(index, group);
          }
  
          return this;
      }
docs: Replaces the current node with the given one.
  <p>
  Can be used to modify the tree.
  
  @param group
  the replacement node
  @return a new groupzipper with a replaced current node