the API in some ways comes from that of Hibernate, which does actually mutate in place (see http://docs.jboss.org/hibernate/orm/3.3/reference/en-US/html...). I felt that keeping the existing object un-mutated is a lot more intuitive. I think the names that you choose for the methods do make a difference, e.g. Hibernate is saying "add()" which for whatever reason seems to imply "mutation" to me, not sure why.
Having used hibernate extensively (and considering many of it's features remain completely unmatched in any other ORM I've encountered), I can say that I strongly dislike the idea of mutating the query in place. There's huge advantages to each chain returning an independent and valid query that can be executed, referenced, and added to. I don't see any upside to the Hibernate approach here, and it violates "Prefer Immutability".
