A tree pattern of the form [phi]([A.sub.1], ..., [A.sub.k])/A conforming to an XML schema S = (r, L, Ter, p) is an XML functional dependency (XFD) over S.
Satisfaction of an XFD is defined against an canonical instance of XML schema.
Let S be an XML schema, D be a canonical instance of S and f = [phi]([A.sub.1], ..., [A.sub.k])/A be an XFD on S.
Then the XPath-oriented XFD [phi]([A.sub.1], ..., [A.sub.k])/A corresponds to the following path-oriented XFD defined in :
--it is easy to check, using only the XPath semantics, whether an XML tree satisfies the XFD or not (see below),
--this form of XFD can be used to generate an XQury program performing some transformation operations (see the next section).
An XFD f can be interpreted as XPath expression, i.e.