how to reproduce
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1) create a test including algebra question type
2) solve test as a student
3) as a teacher click on results/statistics
--> error message is shown
Notice: Undefined property: qtype_algebra_parser_bracket::$_sign in /data_1/www/html/moodle/question/type/algebra/parser.php on line 1339

Notice: Undefined property: qtype_algebra_parser_bracket::$_sign in /data_1/www/html/moodle/question/type/algebra/parser.php on line 1339

Notice: Undefined property: qtype_algebra_parser_bracket::$_sign in /data_1/www/html/moodle/question/type/algebra/parser.php on line 1339

Notice: Undefined property: qtype_algebra_parser_bracket::$_sign in /data_1/www/html/moodle/question/type/algebra/parser.php on line 1339

Notice: Undefined property: qtype_algebra_parser_bracket::$_sign in /data_1/www/html/moodle/question/type/algebra/parser.php on line 1339

Notice: Undefined property: qtype_algebra_parser_bracket::$_sign in /data_1/www/html/moodle/question/type/algebra/parser.php on line 1339

Notice: Undefined offset: 0 in /data_1/www/html/moodle/question/classes/statistics/responses/analysis_for_subpart.php on line 84

Fatal error: Call to a member function count_response() on a non-object in /data_1/www/html/moodle/question/classes/statistics/responses/analysis_for_subpart.php on line 104

remark
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Moodle 3.0.4