With the deffunction construct, new functions may be defined directly in
XSI. Deffunctions are equivalent in use to other functions in XSI.
The only differences between user-defined external functions and
deffunctions are that deffunctions are written in XSI and executed
by XSI interpretively and user-defined external functions are written
in an external language, such as C, and executed by XSI directly.
Also, deffunctions allow the addition of new functions without having to
recompile and relink XSI.
The syntax of the deffunction construct is:
(deffunction <name> [<comment>]
(<regular-parameter>* [<wildcard-parameter>])
<action>*)
<regular-parameter> ::= <single-field-variable>
<wildcard-parameter> ::= <multifield-variable>