Russian praepositions - changing forms rule

[IohannesIohannium]

(Should be coded in separately in relevant games)
Different forms:

• в - во: Second form is used with consonant clusters starting with в, ф, мн. See if this should have its own ru_class letter.
• о - об - обо: Second form used with words starting in non-iotated vowels. Third form with consonant clusters as ditto.

Different praepositions:

• в/на/к: к triggers use of dative, other two of accusative. к is used with animate nouns (I am going to the Celts), в with most other countries (I am going to France), на with islands and peninsulas (I am going to Cuba). Yet to know how to trigger the use of dative.
• из/с: all use genitive. из corresponds to both к and в (I come from France/from the Celts), из to на (I come from Cuba)

ru_class possible update:

Class	towards …	about …	from …
ru_[x]v	в …	о …	из …
ru_[x]vb	в …	об …	из …
ru_[x]vo	во …	обо …	из …
ru_[x]n	на …	о …	с …
ru_[x]nb	на …	об …	с …
ru_[x]no	на …	обо …	с …
ru_[x]k	к …	о …	из …
ru_[x]kb	к …	об …	из …
ru_[x]ko	к …	обо …	из …