The d2o_librarian will fail when mixing different MPI comms
Every local librarian instance on a node of a MPI cluster just increments its internal counter by one when a new d2o is registered. This gets out of sync, when only a part of the full cluster is covered by a special comm.
?Possible solution: The individual librarians store the id of 'their' d2o and communicate a common id for their dictionary. Con: Involves MPI communication.