(line 0 -1/2 0 1/2)

                    (0,1)
       +--------------*--------------+
       |			     |
       |			     |
       |			     |
       |	      * (0,1/2)  <- (x1,y1)
       |	      |              |
       |	      |              |
       |	      |              |
(-1,0) *              |              * (1,0)
       |	      |	             |
       |	      |	             |
       |	      |	             |
       |	      * (0,-1/2) <- (x0,y0)
       |			     |
       |			     |
       |			     |
       +--------------*--------------+
                    (0,-1)