Pure Functions
A pure function is one whose returned value depends only on its arguments (i.e., it possesses referential transparency), and it does not have any side effects.
(define (plus3 x)(+ x 3)) ; a pure function
(define (plus3-with-y x) ; not a pure function
(define (plus3-with-z x) ; not pure unless
(+ x z) ) ; z is constant