Solutions based on predicates, filters and custom iterators/comparators are nice, but they don't provide analogue with database index. For instance: I would like to search collection of cats different ways: by sex and age and by age, so it looks like two indexes:
1) [sex, age]
2) [age].
Values, accessed by that indexes could be hashed, to implement fast lookup, without iterating the whole collection. Is anywhere there such solution?
