Closed question formats such as multiple choice questions, in which the answers are merely selected from predefined options, are often not effective in electronic self-tests with mathematical tasks. The question type STACK for the activity "Quiz" offers a remedy for this problem. STACK uses the computer algebra system "Maxima" to evaluate the mathematical expressions that students enter as answers. The input can be for example a variable, a function term, an equation, a matrix or a set. Teachers can create structured templates for a question, based on which different versions of the question are randomly generated and displayed. Maxima uses tests to prove the mathematical properties of the answer input. Based on the properties, feedback is generated with the help of an answer tree and returned to the students as assistance. The question type can be used in all RWTHmoodle course rooms from summer semester 2020 onwards within the activity "Quiz". As an introduction to STACK the RWTHmoodle support provides an own demo course room for STACK, which contains introductory videos, links to the STACK documentation and numerous sample questions and quizzes. To get access to the demo course room and for general questions about STACK please contact the RWTHmoodle support.