Printing example in gtkmm tutorial

[Igor Gorbounov <igorbounov topazelectro ru>]

It seems like something is missing in printing example
(both simple and advanced).
It doesn't print at all, just waits for something. The program
never enters the on_draw_page() callback.
I've added this method to PrintFormOperation class:
"virtual bool on_paginate(const Glib::RefPtr<Gtk::PrintContext>& context);"
which is:
"bool PrintFormOperation::on_paginate(const 
Glib::RefPtr<Gtk::PrintContext>& context)
{
   return true;
}"

And now this example program prints, previews, and prints to pdf-file.
So what is this "on_paginate" function for, and what is its correct use?
   Igor Gorbounov