One thing surprised me: I was expecting a HashedSequence would also be responsible for triggering hash calculation for certain elements. A getter for the hashCode of element x which would look up if we have the hashCode already and if not trigger calculation. This lazy way of calculation hashes may reduce our resource demand on huge contents. Maybe, on huge contents we switch to a diff which only needs the hashes for a certain window on the Sequence. Ok, ok, that's far away in the moment - but a HashedSequence which really only a dumb store will prevent us to do so.