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You can apply Schematron Quick Fixes over nodes from referenced documents (using XInclude or external entities), and you can access them as nodes in your current document.

Also, you can apply the Quick Fixes over other documents using the doc() function in the value of the @match attribute. For example, you can add a new key in the keylist.xml file using the following operation:
<sqf:add match="doc('keylist.xml')/KeyList" target="Key"
 node-type="element" select="Key2">