write-only properties? (was Re: window border)

[Joe Smith <jes martnet com>]

Once upon a time, muppet wrote:

> > Muppet, it seems that many of the widgets don't implement properties.
>
> actually, they do.
> ...
> > Gtk2::Container properties
> >  "border-width"         gulong               : Read / Write
> >  "resize-mode"          GtkResizeMode        : Read / Write
> >  "child"                GtkWidget            : Write
>
>   $container->set (border_width => 4);
>   $widget = $container->get ('child');

So, since the 'child' property is not readable, that last line won't 
actually work, right?

I was just playing with this and got:
-------------
use Gtk2 -init;
$btn = Gtk2::Button->new( 'Click me' );
$btn->get('child');
-------------
(x.pl:6824): Gtk-WARNING **: gtkcontainer.c:874: invalid property id 3 
for "child" of type `GParamObject' in `GtkButton'

Is this a different 'child' than the label added to the 
button/container?  Ok, yeah, that must be what's befuddled me.

What's the point of a write-only property?

<Joe