** Using WeBWorK **

You can use

    ANS(Compute(",")->cmp(checker => sub {
      my ($correct,$student,$ans) = @_;
      return $correct->isParallel($student);
    }));

to get a list of vectors where the student can enter parallel vectors. This uses a custom checker for the individual entries in the list which returns true when the student answer is parallel to the correct one.

I believe that there is a special basic_cmp in pg/macros/PGmorematrixmacros.pl that might do what you need, though it is not MathObject based. See the POD documentation for that macro file for details.

Davide