Make the program terminate when no items are left to be learned

An item is considered learned once its score goes positive.
The program now runs until no items have a negative score.
If there are fewer than 10 items with a negative score
when it starts, it will introduce some new items from the
pool of items with a score of 0, (otherwise these items
will not be presented).

But items that do have positive scores are always available
to be presented for reinforcement, (and will also go into
the pool of words to be learned before the program terminates
if they are missed).