Types of morphological alternations in the Russian language:
- V -> V:
молола -> мелют; - V -> #:
тёрла -> трут; - # -> V:
гнала -> гонят; - C -> C:
бегу -> бежишь; - C -> CC:
любят -> люблю; - CC -> C:
густой -> гуще; - СС -> CC:
мелкий -> мельче.
Effectively, this means that the edit distance between different stems shouldn't be more than 1.