Log Message: 
Allow parser "constants" to be (in reality) formulas. That way, for
example, you can defined T, N and B to be the formulas for the unit
tangent, normal and binormal of a space curve (as functions of "t")
and the student can write anwers in terms of T, N and B (without
having to compute them).
You could have created FUNCTIONS T(t), N(t) and B(t), and added them
to the parser to get almost the same effect, but the students would
have had to include the "(t)". (If you want that, go ahead and do it
that way.)
