** Using WeBWorK **

Never mind, I figured it out! If anyone else wants this the solution I found that is working is using the studentsMustReduceUnions flag. Here is a working example:

###########################
# Initialization

DOCUMENT();

loadMacros(
"PGstandard.pl",
"MathObjects.pl",
"PGML.pl",

);

TEXT(beginproblem());

##########################
# Setup
Context("Interval");

$a = Interval("{1}U(2,5)U{6}");

BEGIN_PGML

the interval [`[$a]`] is [_]{($a)->cmp(studentsMustReduceUnions => 0)}

END_PGML


COMMENT('Uses PGML.');

ENDDOCUMENT();