Custom Grader with Numerical Input

[sp...@g.harvard.edu]

   I am working on a precalculus course with HarvardX. I am wondering if it is possible to write a custom grader for edX problems that utilizes the formula-response equation parser/evaluator.

   In particular, I want to ask a question like "Simplify b^3 * b^6". I want to accept a response like "b^9" but not "b^3 * b^6" or . To grade responses in this way, I would need to grade in a two-step process. First, we would need to analyze the response as a string (with a custom Python grader) to check that the response is simplified into the correct format (e.g. count the number of "^" characters). Second, we would need to use edX's formula-response engine to check if the formula evaluates to the right thing for different values of "b" (and so students can preview their response).

   I can see in the documentation how to do each of these steps individually (e.g. here and here), but I can't figure out how to combine them. In particular, I don't know how to access the equation parser/evaluator that is used in formularesponse directly from a Python script. Any help you can provide would be appreciated!