[gtkmm] Gtk::Toolbar_Helpers::RadioElem

[RMM <morelli cs utah edu>]

I'm a little puzzled what the intended paradigm is for radiobuttons
in a toolbar,  i.e. 
Gtk::Toolbar_Helpers::RadioElem
I must be missing something here.  This class is just a struct,  so
everything about toolbar radio buttons must be controlled in an
external,  non-OO way.  But where?  I can't find any methods
for setting or querying the state of a toolbar radio button anywhere.  
Which one in a group is initially set?  Why do these radio buttons 
have a different interface and paradigm from ordinary radio buttons?
Is there a way to attach information to the signal generated by a 
click?  The only thing I can see to do is to bind a different callback 
to each toolbar radio button and make some assumptions about how
these buttons get set and unset.

The reference page on this class is not very illuminating and the
example in the tutorial is unhelpful (a toolbar with three radio 
buttons that don't do anything).  I suppose I could consult the source 
code,  but it'd be easier if someone here enlightens me.  

Thanks