(define dg (lambda (x)
 (let ((im (lambda (c)(if (immutable? c) "immutable" ""))))
(cond
  ((boolean? x) (list "boolean:" x))
  ((char? x) (list "char:" x))
  ((null? x) "nil")
  ((symbol? x) (list "symbol:" x))
  ((vector? x) (list (im x) "vector:" x))
  ((pair? x) (list (im x) "pair:" x))
  ((string? x)(list (im x) "string:" x))
  ((procedure? x) (if (parameter? x) "parameter procedure"
      (list "procedure of" (procedure-arity x) "args")))
  ((number? x) (list "number:" x))
  ((port? x) (if (input-port? x) "input port" "output port"))
; stuff below for MzScheme
  ((tcp-listener? x) "tcp-listener")
  ((hash-table? x) (im x) "hash table")
  ((promise? x) "promise")
  ((box? x)(list (im x) "box:" ))
  ((namespace? x) "namespace")
  ((void? x) "void")
   (#t (list "??" x))))))