Here is what a book looks like. The WeBWorK "var" element appears in the RELAX-NG schema as a child of many elements, but almost always as a descendant of a "p" element or a "cell" element. As an element that is only relevant for a WeBWorK problem, occurrences of "var" must be within a "webwork" element. A Schematron rule will check on these two situations.