This paper discusses the nature of mathematical assessment, and compares paper-based learning materials with current online assessments. In the light of this comparison we propose a data structure for representing multi-part mathematical questions, which is a direct development of that currently implemented in STACK. 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. The key difficulty is of increasing the richness of the question types so that they better reflect the needs of question authors without increasing the complexity to a point where they become effectively impossible to produce.