er.scm

(define-syntax swap!
  (er-macro-transformer
    (lambda (form rename compare?)
      (let ((a (cadr form))
            (b (caddr form)))
        `(,(rename 'let) ((,(rename 'value) ,a))
           (,(rename 'set!) ,a ,b)
           (,(rename 'set!) ,b ,(rename 'value)))))))