GtkInvisible - a subclass of GtkWidget or GtkBin?

[Damon Chaplin <damon karuna freeserve co uk>]

The GtkInvisible widget has GtkBin at the start of its struct, yet uses
GTK_WIDGET_TYPE when creating its type:

   invisible_type = gtk_type_unique (GTK_TYPE_WIDGET, &invisible_info);


Is it meant to be a subclass of GtkWidget or GtkBin?

(GTK+ 1.2.0)

Damon