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  hidewindow                                                            WNDWC
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  Function       Hides the current top window on the CRT.
  Syntax         void hidewindow(void);
  Remarks        The window is saved in RAM and the underlay is restored on
                 the screen showing the new top window.  If NOHIDEMODE has
                 been set for this window, the procedure is ignored.  The
                 windows are saved with the border, but without any shadow.
  Return value   None.
  Screens        All video pages.
  EOS            Altered to (wrow,wcol) before hiding, then restored to the
                 new top window.
  Heap           Temporarily uses (underlay size * 1.5).
  Upon return    Forced to write to CRT.