Nested Keep_alive_together_engraver

[Julian Santander]

Trying from a different side.

Say I have:

   - Staff A
   - Group 0 of
   - Group 1 of
         - Staff B
         - Staff C
      - Group 2 of
         - Staff D
         - Staff E
      - Group 3 of
         - Staff B+D
         - Staff C+E

How should I configure the Keep_alive_together_engraver and the
VerticalAxisGroup.remove-remove-empty VerticalAxisGroup.remove-layer to
achieve the following behaviour:

   1. Staff A will be removed if empty.
   2. Staff B and Staff C will be removed if *both* are empty
   3. Staff D and Staff E will be suppressed if *both* are empty.
   4. Staff B+D and Staff C+E will be suppressed if* both *are empty.
   5. Group 3 can replace Group 1 and Group 2.

I'm assuming I need group 0 in order to implement Group 3 replacing Group 1
and Group 2.

I've seen references that in the past there was a group label setting, but
that was replaced in favour of the 'below and 'above settings of the
remove-layer, but I'm unsure about how that could be used to achieve what I
need.

Thanks in advance.

Julian