Checks Shift keys
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 Syntax

        GT_isShift() --> lRet

 Arguments:

      None

 Returns:

      lRet    -       TRUE if Shift is being pressed, otherwise FALSE

 Description:

      Checks status of the Shift Key

 Examples:

      if GT_isShift()
          ? "Get your grubby fingers off one of the shift keys."
      endif

 Source: SHIFT.ASM

 Author:  Andy M Leighton