I think it could be interesting to have to built-in index() function work on many-to-many Map.
For example :
I have an object, say Article, with some text fields that can be translated in many langages.
In Article, I declare a map whose key is Langage Object and value is an object with all my translatable fields.
In some cases, I want make a query on Article and order it according to one of my translatable field and for a given langage.