Rectangle procedure

  DECLARATION:  Rectangle(x1, y1, x2, y2 : Integer);

      PURPOSE:  Draws a rectangle using the current line style and color

         UNIT:  Graph

      REMARKS:  A rectangle is drawn with the upper left corner at (x1,y1)
                and the lower right corner at (x2,y2).  The rectangle will
                be drawn using the current values as declared by SetLineStyle
                and SetColor.  Use SetWriteMode to indicate whether the
                rectangle should be copied or XOR'ed to the screen.

 RESTRICTIONS:  Must first be in graphics mode.