[gtk-list] Gtk builder with a single ui described in multiple files

[Fabrizio Pollastri <f pollastri inrim it>]

Hello,

I have an ui described in my_ui.ui file so I build it with

self.UI = Gtk.Builder()
self.UI.add_from_file('my_ui.ui')

In this ui, there is one widget that is a placeholder for another 
complex widget that is described into another file 'widget_for_my_ui.ui'.

How can I tell to Builder to put widget_for_my_ui.ui into the first ui 
at the placeholder position?

Thanks in advance for any help,
Fabrizio