** Using WeBWorK **

Is it possible to manipulate answers students given to webwork problems?
From the examples I am looking at in the library, I can't see how to do this.

It would be very convenient, if instead of just being able to compare the students answer with a given answer, I apply some function to the answer they enter first. 
E.g., I am trying to write a problem where the student should enter the gcd of two numbers a,b, and also u,v such that au + bv = gcd(a,b).
Since u,v are not uniquely determined, it would be much nicer if I could just
take the values u,v which the student enters, and compute au + bv, and compare this with the value of the gcd.  That way, I don't need to bother telling the student to put u in the range 0 to b, and also it ought to be easier to do this test than to get the webwork to actually calculate some values for u, v.
I can imagine plenty of other cases where it would be nice to manipulate answers, so help on how to do this would be great.