A variable definition binds one or more identifiers and specifies an initial
value for each of them.
The simplest kind of variable definition
takes one of the following forms:
(define variable expression)
(define (variable formals) body)
Formals are either a
sequence of zero or more variables, or a sequence of one or more
variables followed by a space-delimited period and another variable (as
in a lambda expression). This form is equivalent to
(define variable (lambda (formals) body)).
(define (variable . formal) body)
Formal is a single
variable. This form is equivalent to
(define variable (lambda formal body)).