The MathDox Player plays interactive mathematical documents. Some of these documents are exercises, for which the user has to enter a mathematical answer. To give detailed feedback it is useful to be able to perform tests on the answer or to obtain a part of the answer to check if the answer was partially correct (for example the nominator of a fraction). Furthermore it is useful to be able to construct new expressions based on these partial expressions for customized feedback. In the presentation we plan to show some examples of how this can be done using OpenMath, XPath and Jelly in MathDox documents.