IOUs can not be edited or deleted 25 days after they have been entered.
This restriction helps avoid confusion for transactions that are old enough to
be accepted by all participants as correct. If you made a wrong entry, it's very
likely to be noticed within 25 days. If not, it's highly recommended you add a
new transaction to correct the difference, instead of deleting the old one.
Editing or deleting old IOUs is prone to abuse by people
who want to mess up other's balances, and hence we do not allow it.