Russian plural

Software doesn't recognize it. It just acts differently when less than usual number of forms are given (which is something that makes it hard to unify mediawiki rules with other systems).

I have tried to write this up on Plural#Alternative_ruleset. As usual, I would be grateful if this could be checked for accuracy and usefulness. I have also added an extra column to the table of Mediawiki plural rules to carry a note on which languages have the additional ruleset.