Focus chain ineffectual

[Jeffrey Barish <jeff_barish earthlink net>]

I'm trying to set the focus chain.  I have 3 widgets that I want to chain,
so I specified them as the arguments of the set_focus_chain method on the
top-level window.  I expect focus to move from one to the next as I hit
tab.  Instead, focus moves to many widgets not in the chain in addition to
some that are in it.  It seems as if the set_focus_chain call is not doing
anything.  can_focus is set on all three widgets.  Am I not allowed to call
it on the top-level window?  Are the widgets required to be immediate
descendants (the manual says only that the widgets should be "descendants",
which these are)?  Is there a different explanation?
-- 
Jeffrey Barish