Fix #17

[cdemulde]

This should fix #17, at least it works for the minimal working example provided (#17 (comment)) and the actual code I have.
To be checked though, because I'm not entirely sure of the behaviour of self.last_index and self.reference_counter.

[nilp0inter]

Hi, thank you for your contribution. Unfortunately I won't be able to review the issue and the patch until next week. Thank you for your patience.

[cdemulde]

Hi @nilp0inter,

Have you had the opportunity to consider my above adjustments? Could you let me know whether or not you intend to include them in a new version of experta? Depending on your answer, I code adjust the code of the project I am working on in a future-proof way.

Thanks!