How to detect optional parts?

```
Say, we have rule "a? b? c". Now we parse "ac" and "bc". HOW I can determine 
which optional part was specified? In 'results' array in both cases will be two 
items. Detecting by Text? No way, text can be too big and strictly speaking 
should not be involved in this procedure.
Any suggestions?
```

Original issue reported on code.google.com by `2daite2@gmail.com` on 21 May 2015 at 1:35
