PPT Slide
The built-in function defun allows us to define new functions.
( defun <name> <arglist> <body> )
<name> is a symbol, naming it
<arglist> is a list of its formal parameters
<body> is an expression representing its body
( defun 2+ ( x ) ( 1+ ( 1+ x ) ) )
( defun cadr ( L ) ( car ( cdr L ) ) )