Since a lot of time a big cache block is used to avoid rebuilding the navigation menus of the website at every page view, it really makes sense to add a max_depth parameter that goes hand-in-hand with the subtree_expiry.

This way if the cache-block displays all the chidren of root node

• of type folder
• which are direct children or grand-children
  using a subtree_expiry=2 subtree_expiry_depth=2 will make creation of an efficient cache block a piece of cake.

It also enables making a cache block depending on a single node and not on its children.