** Using WeBWorK **

First, MathObjects are targeted around single-line answers, so there isn't much support for multi-line answers, as you have found. The ArbitraryString context that you have been using wasn't intended for multi-line input, and so it fails in severals ways when these are used.

Second, the correct answer used to be displayed as a string, but now it is the result of a TeX expression. That is a fairly recent change, and it was added on top of the MathObjects library in what was perhaps not he bet possible way (I am going to need to look into straightening that out).

In any case, the contextArbitraryString.pl file can be modified to do what you are looking for. I have made the modifications, and attach the changed file. Put it in your course's templates/macros file (or in the system pg/macros directory), and then you can use something like:

    DOCUMENT();

    loadMacros(
      "PGstandard.pl",
      "contextArbitraryString.pl",
      "problemPreserveAnswers.pl",
    );

    TEXT(beginproblem());
    
    Context("ArbitraryString");
    
    $a = Compute("Line 1~~nLine 2");
    
    BEGIN_TEXT
    Enter your code:$BR
    \{ANS_BOX(0,5,20)\}
    END_TEXT
    
    ANS($a->cmp);
    
    ENDDOCUMENT();

The ArbitraryString context will now properly handle multi-line input and correct answers. It renders the answers via an array (so that it can be rendered by TeX). The problemPreserveAnswers.pl file is loaded above in order to allow student answers to include characters like $ and @ that would not be properly saved by WeBWorK otherwise.

Note that the linebreaks that are returned as part of the answer can vary from platform to platform, so Mac browsers may give you carriage returns, and Windows may give you carriage return-linefeed pairs. This is taken care of in the ArbitraryString comparison routine, but if you do your one custom answer checker, you may need to normalize the line breaks yourself.

Davide