A function call in XPath 2.0 takes the form
F ( arg1, arg2, ...). In general,
the function name is a QName. A library of core functions is defined in the XPath and XSLT
specifications (in versions 2.0, 3.0, and 3.1). For details of these functions, including
notes on their implementation in this Saxon release, see the Function library. Additional functions are available (in a special
namespace) as Saxon extensions: these are also listed in the Function library, with some further information in the Extensions section. Further functions may be
implemented by the user, either as XSLT stylesheet functions (see xsl:function), as XQuery
functions, or as Java extension functions (see the Extensibility mechanisms section).
XPath 3.0 introduces dynamic function calls:
EXPR ( arg1, arg2, ...) where
EXPR is an expression that evaluates to a function item.