Different *types* of subtrees usually get the same ett, since it
preserves attributes like whether it is expanded or not and the such.
If I have N subtrees, one for each extension, with N unknown at compile
time, displayed one after the other at the same time, I suppose I cannot
share their etts.
Am I right?
If so, this brings me back to the original problem: how can I register a
variable number of etts in advance?