Ds allow you to define new functions in CLIPS
directly. In previous versions of CLIPS, the only way to have
userdefined functions was to write them in some external language,
such as C or Ada, and then recompile and relink CLIPS with the
new functions. The body of a deffunction is a series of expressions
similar to the RHS of a rule that are executed in order by CLIPS
when the deffunction is called. The return value of a deffunction
is the value of the last expression evaluated within the deffunction.
Calling a deffunction is identical to calling any other function
in CLIPS. Deffunctions are covered comprehensively in Section
7.