ir.scm

(define (subst old new tree)
  (cond ((eq? old tree) new)
	((pair? tree)
	 (cons (subst old new (car tree))
	       (subst old new (cdr tree))))
	(else tree)))

(define-syntax x2y
  (ir-macro-transformer
   (lambda (form r compare)
     (let ((tree (cadr form)))
       `',(subst 'x 'y tree)))))

; (x2y (x x y (x ((x z)) z)))
; expect: (y y y (y ((y z)) z)
; => (x x y (x ((x z)) z))