In this talk I will describe a computer aided assessment system for
mathematics known as STACK, a System for Teaching and Assessment using a
Computer algebra Kernel. STACK makes use of the computer algebra system
Maxima for a variety of tasks, the most important of which is establishing
mathematical properties of student's answers. We discuss STACK with a
focus on (i) the data structure used to represent questions, (ii) how
teachers write questions of their own and (iii) the characteristics needed
of a CAS for this application.