Gtk... collapsable toolbars?

[Douglas Ian Linder <linderdi cs curtin edu au>]

Hello,

I've seen an example (compiled. :( ) of a gtk application with toolbars
like those seem in mozilla; ie. on the far left is a left textured bar
which when clicked flattens the toolbar and makes it disappear.

How is this affect achieved? Is it just another button added to the 
toolbar that just sets the view state of the elements on the toolbar
to hidden or is this actually a specific type of toolbar widget?

If the case is the latter, what widget? Or how do I make a normal
toolbar behave in this way?

cheers!
Doug.