Figure 1: BPE transformation

Original data:     A   B   A   B   C   A   B   C   D
                   |   |   |   |   |   |   |   |   |
Change AB to E:     ---     ---    |    ---    |   |
                     |       |     |     |     |   |
Intermediate data:   E       E     C     E     C   D
                     |       |     |     |     |   |
Change EC to F:      |        -----       -----    |
                     |          |           |      |
Compressed data:     E          F           F      D