You can do this too, but it's not quite as simple. You want to change the "problem-grader"
the tiny program that decides how to grade a particular problem.
There are some examples at:
and in particular
does exactly what you want (click on source to view the source code)
The examples may not yet be perfect, we haven't used this feature very much, so there
may still be some glitches, but I think you will be able to get it to do what you want.
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