get interface pointer, how?

[Ian Zimmerman <itz buug org>]

Hi, I read both the GLib/GObject reference manual and mathieu gnu org's
Maman/Papa tutorial, but I still can't figure out the most basic thing
about interfaces: how to actually GET one?  I am not creating or
implementing one, I just want to use it.

Specifically, GListStore is said to implement GtkTreeSortable.  So,
given a GListStore*, how do I get the GtkTreeSortable*?

And, doesn't this deserve to be documented better than the specifics
of interface registration and implementation?  Especially since GtkCList
is deprecated so GtkTreeView is the only approved way of creating sortable
lists ...

Thanks, Ian

-- 
Nothing can be explained to a stone.
Or to a stoned person, either.