Trying to add an
  [optional] interface XFoo;
to a service which is already published (i.e. has the "published" keyword),
results in an error message from the IDL compiler, saying
  an unpublished entity cannot be used in the declaration of a published entity:
  '<interface>'

As agreed, we should relax this that adding *optional* unpublished entities
would still be allowed.
The alternative for the IDL designer would be to document this additional,
optional interface in the comments only, which has the disadvantage that it
would be reflected nowhere - in particular not in the UNO type repository -
except in the generated IDL documentation (which is nice, but not sufficient).