SBCENTER()

  Short:
  ------
  SBCENTER() Centers box coordinates

  Returns:
  --------
  Nil

  Syntax:
  -------
  SBCENTER(@nTop,@nLeft,@nBottom,@nRight)

  Description:
  ------------
  Dimensions nTop,nLeft,nBottom,nRight are passed by
  reference.

  They are changed by SBCENTER to dimensions that are
  centered on the screen.

  Examples:
  ---------

   nTop    := 0
   nLeft   := 0
   nBottom := nRows+2
   nRight  := nMaxwidth+2

   SBCENTER(@nTop,@nLeft,@nBottom,@nRight)
                  // center the coordinates

   cBox := makebox(nTop,nLeft,nBottom,nRight)

  Source:
  -------
  S_BOXES.PRG