[plt-scheme] basic question about dynamic interaction

Is there documentation describing how to dynamically
interact with DrScheme?

example: under Version: 299.22-cvs17nov2004 I run
.../collects/sgl/examples/gears.ss
Obviously the program is event-driven and even though
the prompt is displayed while the gears turn, nothing
can be accessed because nothing is in scope except for
the top-level definitions. Only if I instrument
everything with a global definition can I change it
during execution.
Do I *have* to use emacs to submit the entire
redefinition of "(on-paint)"?
Why can I not simply highlight the region and 
click on "re-evaluate" or something, without stopping
the program? 

(define mycount 1)

(define/override (on-paint)
      (when gear1
	(when (>= (- (current-seconds) sec) mycount)
	  (send (get-parent) set-status-text (format "~a fps"
(/ (exact->inexact frames) 5)))
	  (set! sec (current-seconds))
	  (set! frames 0))
	(set! frames (add1 frames))

> (set! mycount 10)



		
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