If you are reading this inside of Info in Emacs, you can try out the
multiply-by-seven function by first evaluating the function
definition and then evaluating (multiply-by-seven 3). A copy of
the function definition follows. Place the cursor after the last
parenthesis of the function definition and type C-x C-e. When you
do this, multiply-by-seven will appear in the echo area. (What
this means is that when a function definition is evaluated, the value it
returns is the name of the defined function.) At the same time, this
action installs the function definition.
(defun multiply-by-seven (number) "Multiply NUMBER by seven." (* 7 number))
By evaluating this defun, you have just installed
multiply-by-seven in Emacs. The function is now just as much a
part of Emacs as forward-word or any other editing function you
use. (multiply-by-seven will stay installed until you quit
Emacs. To reload code automatically whenever you start Emacs, see
Installing Code Permanently.)