gtk:application-add-window (application window)

application -- a gtk:application instance
window -- a gtk:window widget

Adds a window to the application. This call can only happen after the application has started. Typically, you should add new application windows in response to the emission of the "activate" signal. This call is equivalent to setting the application property of the given window to the given application, that is:

(gtk:application-add-window application window)
== (setf (gtk:window-application window) application)  

Normally, the connection between the application and the window will remain until the window is destroyed, but you can explicitly remove it with the gtk:application-remove-window function. GTK will keep the application running as long as it has any windows.