Tk packing problem, in_ parameter fails?

[Jeff Epler]

The pack(n) manpage has this to say:
RESTRICTIONS ON MASTER WINDOWS
    The master for each slave must either be the slave's parent (the
    default) or a descendant of the slave's parent.  This restriction is
    necessary to guarantee that the slave can be placed over any part of
    its master that is visible without danger of the slave being clipped
    by its parent.

As far as I can see, this "restriction" is met by your code.  The
slave is "wdg", its parent is "root", and "tl" is a descendant of
"root", the slave's parent.  The restriction text should also mention
that the slave must be within the same toplevel as the master.

However, this does not work in "wish" either:
    $ wish
    % toplevel .t
    .t
    % label .l
    .l
    % pack .l -in .t
    can't pack .l inside .t
thus, Python can't do anything to ease this restriction.

Jeff