modules, but thunk? requires loading (system vm debug).  This is in
  ;; contrast to the open-coded version of dynamic-wind, which does
  ;; currently insert an eager thunk? check (but often optimizes it
  ;; out).  Not sure what the right thing to do is here -- make thunk?
  ;; callable before modules are loaded, live with this inconsistency,
  ;; or remove the thunk? check from the compiler?  Questions,
  ;; questions.
  #;
  (unless (thunk? out)
    (scm-error 'wrong-type-arg "dynamic-wind" "Not a thunk: ~S"
               (list out) #f))
  (in)
  ((@@ primitive wind) in out)
  (call-with-values thunk
    (lambda vals
      ((@@ primitive unwind))
      (out)
      (apply values vals))))

(define (with-fluid* fluid val thunk)
  "Set @var{fluid} to @var{value} temporarily, and call @var{thunk}.
@var{thunk} must be a procedure of no arguments."
  ((@@ primitive push-fluid) fluid val)
  (call-with-values thunk
    (lambda vals
      ((@@ primitive pop-fluid))
      (apply values vals))))

(define (with-dynamic-state state thunk)
  "Call @var{proc} while @var{state} is the current dynamic state object.
@var{thunk} must be a procedure of no arguments."
  ((@@ primitive push-dynamic-state) state)
  (call-with-values thunk
    (lambda vals
      ((@@ primitive pop-dynamic-state))
      (apply values vals))))