It often arrives that I want to know when a card is going to be
flushed and in this case change it.
This could be the case if I want to change the scheduler without
implementing a whole scheduler. It simply reads the card history and
change interval and due date.
It's also the case for the "'trigger -> action' rules", which apply
some coded actions when some event occurs. E.g. suspend/unsuspend a
sibling when card become mature/is forgotten.
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