The tape label is saved in a disk file, to which the environment variable
TAPE_LABEL_FILE points to. This label is used to obtain the default
version for the labels that automatically show up.
This default value can be changed, e.g. in the case that an output device
was opened, a few events were written to and then the online system crashes,
such that one wants to overwrite that tape. How this is done is shown below.
However in most cases one wants to use the default value.
If one opens one tape at a time, the tape label will be incremented. If one wants
multistreaming, then all the tapes have to be opened at the same time using
the ``open all closed streams'' button.