:>   macro

;; start or modify looper
(define-macro (:> tag . args)
  (let ((tagtime (string->symbol (string-append (symbol->string tag) "_tagtime"))))
    (if (not (defined? tagtime)) (eval `(define ,tagtime (now)) (interaction-environment)))
    (if (< (eval tagtime) (- (now) (* 2 *second*)))
        (eval `(define ,tag #f) (interaction-environment)))
    (if (and (defined? tag) (closure? (eval tag)))
        `(rmap-loop-runner ,tag ,tagtime modify ,@args)
        `(rmap-loop-runner ,tag ,tagtime start ,@args))))