=item B<--unless-exists>
Will only create the new fields if this field does not exist
Will only create the new fields if this field does not exist.
For instance, if --field='905$a=0/%Y' and --unless-exists='905$a' are provided, a 905$a will be created unless there is already one.
If --unless-exists is not passed, a new 905$a will be created in any case.
=back
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