+- Digit 0 0 | 1
                                                           |            |
                               +- Term 0 2 --- Number 0 2 -+- Digit 1 1 | 2
                               |                           |            |
                               |                           +- Digit 2 2 | 0
                               |                                        |
Expression 0 4 --- Factor 0 4 -+----------------------------- AddOp 3 3 | +
                               |                                        |
                               +- Term 4 4 --- Number 4 4 --- Digit 4 4 | 5