Drill quizzes the user on the problems in the given timelearn.T
task. How the questions and answers are presented is determined by
the 'kind' stored when the timelearn.T database was created. New
learners can be added by calling Register.
The UI is responsible for asking the user a question, and
determining if they got it right. The prob argument will always
point to the problem to learn. 'next' may point to an upcoming
problem.