15.8.2. Details of XQuery Syntax
Virtuoso XQuery uses some syntax extensions. Most visible is an additional notation for qualified names as described above (name is surrounded by "(!...!)" delimiters. An earlier implementation allowed single-line comments started with "#" or "--" continuing to the end of line, this syntax is now obsolete.
The "default namespace declaration" clause is not currently supported, to make the text of XQuery unambiguous. If used, default namespaces must extend element names but not attribute names. Extension function names must be extended as they have non-default namespace prefixes but the names of basic functions should not be extended by the default namespace. Finally, Virtuoso will not preserve any information about used namespace prefixes, so default namespaces will be converted into non-default when the resulting XML entity is printed.