the idea was that snapshots would form independent versions within the session. so the following rules are supposed to apply:

   * if you create a new snapshot, the current state of the session is saved to the new snapshot, but all subsequent saves continue to go to the "mainline"
   * if you do "Save As", the current state of the session is saved to a new snapshot, and subsequent saves will go to that snapshot (i.e. you are now
              working with the saved snapshot)
   * if you load a snapshot or the "mainline", all saves will go to that version until you do something to change that