DEFUN
=====

syntax: (defun <name> ({<argument> ...})
         <documentation-string> ({<instruction> ...}))

DEFUN defines a function.

The documentation-string is only for making the
program more readable. It can be left away!

example:       >(defun mittelwert (x y)
                       (/ (+ x y) 2))
               MITTELWERT
               >(mittelwert 2 3)
               5/2