capi:interactive-pane

What is expected of the TOP-LEVEL-FUNCTION of an interactive-pane?

The following doesn't work:

(defun work ()
  (with-open-stream (s (system:open-pipe "/bin/bash"
                                         :direction :io
                                         :buffered nil))
    (capi:contain (make-instance 'capi:interactive-pane 
                                 :stream s
                                 :top-level-function #'(lambda (interface pane stream)
                                                         (format *debug-io* "~A" (read-line stream)))))))

I would have expected this to form a primitive xterm.

PS: I know about the shell tool. I need something similar for an
application that will connect to an interactive shell like process but
the above is an example that can run anywhere.

-- 
Honest praise, this stony part insisted, was what the bunglers of the world 
heaped on the heads of the barely competent.                -- Stephen King