All XPath implementations must include certain core functions. For particular applications, this function library may be augmented. For example:
XML canonicalization adds the function “here( )”. (See Chapter 9.)
XPointer adds several functions and data types. (See Chapter 7.)
Of course, the various operators that can appear in XPath expressions may also be considered as functions invoked by the operator's syntax rather than the usual functional syntax. XPath does not consider the node tests listed in Section 6.3.3, which look like function calls (such as “text( )”), to actually be functions.
The rest of this section describes the functions in function prototype format. An italicized data type before the function name gives ...