[plt-scheme] Mred - How to know which canvas is active?

Hi all,

I'm using Mred and can't get the active canvas which has keyboard 
focus on.
My interface is like DrScheme's one: i've got a frame containing 2 
editor canvases.

I tried: (has-focus?) which doesn't make it neither does (get-
active-canvas).

Section 2.3 from manual "Mouse and Keyboard Events" is talking 
about (on-subwindow-event)
and (on-subwindow-char), but i didn't figure out how to make them 
work and i'm not sure
that's the one to use.

Any help please?
Thks in advance.