How can write XForms that performs a check in the XML data by invoking XQuery?

[dadiu]

I am new to the XForms technology, and I am finding difficult to achieve the following functionality.My objective is: upon clicking a link or button to edit a resource, typically XML, some checking (by invoking an XQuery for example, checking an element within the resource) has to happen, and based on the return value of the XQuery I need to throw an error dialog or edit window.It would be great if someone guides me through in achieving the above functionality.