VEraseOverlayLine - erases a 1-bit overlay line.
  VEraseOverlayLine(win, ovl, x1, y1, x2, y2)
  Window win;
  int ovl;
  int x1, y1, x2, y2;
  VEraseOverlayLine erases a 1-bit overlay line, in the image window or
one of its subwindows. 
Return Value
  0 - work successfully.
  1 - memory allocation error.
  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.
  ovl - specifies the overlay number to be used to ease the line.
  x1, y1 - specifies the first point of the line.
  x2, y2 - specifies the second point of the line.
Side Effects
  Any other graphics in that overlay on that line will also be erased.
Entry Conditions
  VTurnOnOverlay should be called first. If ovl, x1, x2, y1, or y2 is not 
valid, or VCreateColormap is not called earlier VEraseOverlayLine prints an 
error message to the standard error stream, produces a core dump file and 
exits from the current process.
Related Functions
VDisplayOverlayLine, VEraseOverlay, VTurnOffOverlay.