###########################################################
#
#  Example showing how to use the Parser's function
#  answer checker.
#

DOCUMENT();        # This should be the first executable line in the problem.

loadMacros(
  "PGbasicmacros.pl",
  "PGanswermacros.pl",
  "Parser.pl",
  "parserUtils.pl",
);

TEXT(beginproblem());

###########################################################
#
#   The setup
#
Context('Interval')->variables->add(a=>'Real');
$x = Formula('x'); $a = Formula('a');

$f = log($x-$a);
$I = Formula("(-infinity,a]");

###########################################################
#
#  The problem text
#

Context()->texStrings;
BEGIN_TEXT

Suppose \(f(x) = $f\).
$PAR
Then \(f\) is undefined for \(x\) in the interval(s)
\{ans_rule(20)\}.
$PAR
${BCENTER}
${BSMALL}
To enter more than one interval, separate them by commas.$BR
Use ${LQ}${BTT}infinity${ETT}${RQ} for ${LQ}\(\infty\)${RQ} and
${LQ}${BTT}-infinity${ETT}${RQ} for ${LQ}\(-\infty\)${RQ}.$BR
Enter ${LQ}${BTT}NONE${ETT}${RQ} if the function is always defined.
${ESMALL}
${ECENTER}

END_TEXT
Context()->normalStrings;

###########################################################
#
#  The answers
#
ANS(List($I)->cmp(
  list_type  => 'a list of intervals',  # override these names to avoid
  entry_type => "an interval",          # 'formula returning ...' messages
));
Context()->variables->remove('x'); # error if 'x' is used in answer

$showPartialCorrectAnswers = 1;

###########################################################

ENDDOCUMENT();        # This should be the last executable line in the problem.