a question about begin and character encoding

Hello, I haven't been able to figure out a few questions. Can you help me?

If it's just to satisfy:
```
for i := 0; i < len(siblings); i++ {

d. darts.Check [begin+int(siblings[i].code)] = begin

}
```

Why is the value of begin `begin = pos - int (siblings [0]. Code')`, based on what consideration? Why not start with 0



And why `code = Unicode + 1`

Thank you