;こんな感じ。
;http://pre.plt-scheme.org/plt/doc/reference/mpairs.html
(require scheme/mpair)

(define-syntax cons
(syntax-rules ()
((_ a b) (mcons a b))))

(define-syntax set-car!
(syntax-rules ()
((_ a b) (set-mcar! a b))))

(define-syntax set-cdr!
(syntax-rules ()
((_ a b) (set-mcdr! a b))))

(define-syntax list
(syntax-rules ()
((_ a ...) (mlist a ...))))

(define Qoo (list 3 4))
(set-car! Qoo '(5 6))
(set-cdr! Qoo '(7 8))
(display Qoo) ; => {(5 6) . (7 8)}