MDI introduces some complexities which are readily handled by
  the Clip-4-Win Application Classes.
  
  However, because of MDI's peculiarities and exacting
  requirements, a lengthy discussion is really needed if you
  want to use MDI in a non-OO application.  In outline:
  
  (a)  An MDI application has a main window, called the frame window
       (or, in VO, the shell window).
  (b)  The frame has a client that creates, owns and manages any child
       windows.  You must interact with this client in the appropriate
       manner.
  (c)  The frame or any child can have a menu.  The current child's
       menu is displayed in the frame window.
  (d)  Any minimised (iconised) children appear in the frame, not at
       the bottom of the Windows desktop.  The MDI client will arrange
       this, but you have to co-operate with it.
  (e)  Child windows should generally be modeless.
  (f)  Dialogs should behave as much like child windows and child
       windows as possible.  At times these conflict.  Windows does
       not provide a ready-built MDI dialog, so Clip-4-Win does.