XML Query Requirements include that XML Query...

• must be declarative
• must have closure
• must have an XML syntax
• must be able to perform arbitrary transformations
• must have a human-readable syntax (could be the XML syntax)
• must be able to operate on collections of documents
• must be able to operate on references between and within documents
• must be able to operate on XML Schema datatypes
• http://www.w3.org/TR/xmlquery-req