I am trying to write a problem where the answer is supposed to be a union of intervals with one of the intervals being a singleton. The problem is a bit more complicated, the intervals are randomly generated, but basically I do this:

DOCUMENT(); loadMacros( "PGstandard.pl", "MathObjects.pl", "PGcourse.pl", "PGML.pl", "PGgraphmacros.pl", "AnswerFormatHelp.pl", "PGchoicemacros.pl", ); TEXT(beginproblem()); $i1 = Interval("(-Infinity,-2)"); $i2 = Interval("(-2,1]"); $i3 = Interval("{3}"); $i4 = Interval("[4,5)); $answer = Union($i1, $i2, $i3, $i4); BEGIN_PGML Blah blah instruction Answer: [_______________________]{$answer} END_PGML BEGIN_PGML_SOLUTION Blah blah the answer is [` [$answer] `]. END_PGML_SOLUTIONIn the solution, the answer is displayed correctly, but when I try to enter the answer, I get "Operands of 'U' must be intervals or sets", with the 'U' immediately preceding the '{3}' highlighted.

I tried to enter [3,3] instead of {3}, and it works, but in answer help for Interval, it specifically says that "When an interval contains only one point, enter it as a one point set such as

`{3}`

or as a closed interval such as `[3,3]`

." It seems like only the [3,3] notation works. Is there any flag I can set to make the answer checker accept the {3} notation as well?