* Refactor get_next_coords().

    - I don't see too much point to it. It is working as it is, and it's
    only one function that doesn't affect the rest of the code.

* Create a base class for classes with 'subs' functionality - like
get_nth_sub, add_sub, num_subs, etc. Both ..::Node and ..::Item share it.

    - They have slightly different semantics, and besides - it's not
    too much duplicate code. It would be more trouble than it's worth.

* Future direction: generate the tree once and then be able to render
it for different path_info()/current_host() configurations. (and the
generated tree re-used by all other members).

    - the problem is that the expand() operations and the expanded()
    flag depends on this combination. Thus, the generated tree is
    different for any path_info()/current_host().