Safe With Handler

  ;; From the chapter "Condition system" in "The Nearly Complete
  ;; Scheme48 Reference Manual" by Taylor Campbell:
  ;;
  ;;    *Warning:* `With-handler' is potentially very dangerous.  If an
  ;;    exception occurs and a condition is raised in the handler, the
  ;;    handler itself will be called with that new condition!
  ;;    Furthermore, the handler may accidentally return to an unexpecting
  ;;    signaller, which can cause very confusing errors.  Be careful with
  ;;    `with-handler'; to be perfectly safe, it might be a good idea to
  ;;    throw back out to where the handler was initially installed before
  ;;    doing anything.
  ;;
  ;; The following is what he then suggests.

  ;; ,open handle
  (define (with-handler/safe handler thunk)
    ((call-with-current-continuation
      (lambda (k)
        (lambda ()
          (with-handler
              (lambda (kond propagate)
                (k (lambda () (handler kond propagate))))
            thunk))))))
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