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An extensible system is one that enables extensibility.  Enabling extensibility
means providing non-privileged mechanisms to extend existing objects and to
introduce new objects.  [[UNIX]] is generally not an extensible system as it does
not generally facilitate the hooking of system calls.  For instance, there is
no way to hook into the virtual file system.  This has motivated the introduction
of separate, parallel interfaces by both the GNOME and KDE projects to provide
users a more integrated view of their objects.