Grasple has multiple check methods using the Computer Algebra System (CAS) which can be used within the editor to indicate when the answer of a student should be marked correct/incorrect. A full list of current options can be found in this article.
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This article covers the Defined check. There are two sections in this article:

If you want to check whether an operation can be performed on the answer of a student. The process for this is two-fold. First, you apply the operation on the answer of a student, for example by taking the inverse of the student answer and storing this in a parameter:

Secondly, you check whether this parameter is (or is not) Defined (e.g., has a value) in an answer rule:

The CAS checks whether the variable exists and has a value other than NaN.

Do you want to know more about this check, whether you should use it for your exercise or any of the other checks? Please let us know via the chat icon in the right bottom of the screen!