Cannot read properties of undefined (reading 'getConstant')

[Ben Brawn]

This formerly working question has recently broken. Part (a) of 

https://numbas.mathcentre.ac.uk/question/121693/ben-s-copy-of-broken-indices-expanded-vs-index-form-algebraic/

It has some custom marking code (which I'm sure I borrowed or modified from a demo question) to confirm that the student has written the answer in the form a^4 and that seems to be the issue. 

A few people have copied this question so all of their versions will be broken too atm.

There is a pop-up of errors like "Numbas Error: Error in part Gap 0 custom script mark_answer: Cannot read properties of undefined (reading 'getConstant')" and after that we get "Error evaluating variable mark: Variable all_valid is not defined." on the page.

[Ben Brawn]

For the moment, I've just commented the custom marking out of the live version and I'm just using forbidden strings to avoid the student entering the question in.

[Christian Lawson-Perfect]

This must have been broken for a couple of years at least, unless I'm missing something. Part a uses the old pattern-matching functions, so would have broken in 2019 when the new pattern-matching code was introduced.

I think the easiest way to fix this is to just use the built-in pattern restriction, instead of custom JS marking script.

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