Precondition add-must-be-new-label

Hi,

The add-must-be-new precondition of the LABEL method makes it
impossible to move a label. Moving a label requires a remove
followed by an add, which are two round-trips. Instead of
having the precondition, the postcondition label-must-be-unique
would provide the same data integrity, while avoiding the
extra round-trip.

An additional advantage is that the move would be atomic. A
remove followed by an add is not. The impact of a failure in
between can be dramatic.

Regards,

Werner.
-- 
Werner Donné  --  Re
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