VEraseOverlay - erases a 1-bit overlay image.
  VEraseOverlay(win, ovl, xloc, yloc, width, height)
  Window win;
  int ovl;
  int xloc, yloc;
  int width, height;
  VEraseOverlay erases a 1-bit overlay image within a rectangle in the
image window or one of its subwindows. 

Return Value
  0 - work successfully.
  6 - invalid window ID.
  225 - no overlay created.
  237 - the overlay is not on.
  win - the image window ID or one of its subwindows' ID.
  xloc, yloc - specifies x and y coordinates of the upper left corner of the
        rectangle, relative to the origin of the image window.
  width, height - specifies the width and height in pixels of the overlay.
  ovl - specifies the overlay number to be used to erase the 1-bit image.
Side Effects
  Any other graphics in that overlay within that rectangle will also be
Entry Conditions
  VTurnOnOverlay should be called first. If ovl, xloc, yloc, width, or height 
is invalid, or VCreateColormap is not called earlier, or the overlay image to 
be erased is totally out of the specified window boundary, VEraseOverlay 
prints an error message to the standard error stream, produces a core dump 
file and exits from the current process.
Related Functions
VDisplayColorImage, VDisplayGrayImage, VDisplayOverlay, VTurnOnOverlay, VTurnOffOverlay.