The elembuilder module calls
when the user answers wrong. This is problematic since the percentages
table will show a row with the label "None" since PercentagesTable.update
Using all_keys=True is correct, because this will let the user see the
statistics for questions they nevery have answered correctly.
A user have suggested that we should store the users answer when answering
wrong. I think this is better than storing None, since it will make
elembuilder consistent with the other exercises. But since answering
is "free-form" the user will soon fill up the statistics database with lots
of answers keys that will never be asked. So if we do this, we need to
filter the statistics keys when displaying statistics.
In the current implementation, statistics.py will see all correct and wrong
answers, and assume that using all those keys is ok. Maybe the exercise
need to tell the statistics module which keys to display.