+ <li> #Mdevice, the value must be one of #Mx, #Mgd, and #Mnil.
+
+ If the value is #Mx, the frame is for X Window System. The
+ argument #MDrawWindow specified together with the frame must be of
+ type @c Window. The frame is both readable and writable, thus all
+ GUI functions can be used.
+
+ If the value is #Mgd, the frame is for an image object of GD
+ library. The argument #MDrawWindow specified together with the
+ frame must be of type @c gdImagePtr. The frame is writable
+ only, thus functions minput_XXX can't be used for the frame.
+
+ If the value is #Mnil, the frame is for a null device. The frame
+ is not writable nor readable, thus functions mdraw_XXX that
+ require the argument #MDrawWindow and functions minput_XXX can't
+ be used for the frame.
+
+ <li> #Mface, the value must be a pointer to #MFace.