From the Archives! I found an interesting old page on the W3C which gives a list of various examples of Co-occurrence constraints. I expect it was created by the W3C XML Schemas Working Group to get Use Cases for the XPath features which were added in XSD 1.1.
Some of the Schematron material is outdated, because it is assumes XPath 1 rather than XPath2. But nevertheless, it might be interesting reading for anyone who wants to get a better feel for co-occurrence contraints.