+-------------------------------------------+
| |
+----+---+ +--------+ +--------+ |
| | | | | | |
| R(i-2) | | R(i-1) | | R(i) | |
| | | | | | |
+----+---+ +----+---+ +---+----+ |
| | | |
| | | |
| | | |
| | v+ v-
| |
| | + +--------+ +--------+
| +-----> | | + | |
| + | S(i) | +---+> | S(i+1) |
+-----------------+> | | | |
+--------+ +--------+