Namespaces and XPath expressions


The difficulties induced by the uses of namespaces appear immediately when XPath expressions are involved.

The problem comes from the practical necessity to use prefixes to write legible and understandable XPath expressions.
Of course, those prefixes stand for the namespace URIs that are defined in the XML document to be dealt with,
so that some technical device must be provided to specify the correspondence between prefixes and URIs.

Note that the set of prefixes used in writing XPath expressions is independent of the system used in the XML file,
only the URIs must be tne same. In particular, empty prefixes will be disallowed
(because of the ambiguity between default namespace ans no namespace)
so that at this level default namespaces are to be treated like any other one.

Here are two case studies :