@Override
        public void clear() {
            // remove the content of the backing from the bucket content set
            AbstractBucket.this.content.removeAll(this.backing);
            // then clear the backing
            this.backing.clear();
        }
Class used to wrap the backing sets returned by {@link #getPartition(int)}.

This wrapping prevents add operations, and propagates calls with remove objects
back to the parent bucket.
helper/src/main/java/me/lucko/helper/bucket/AbstractBucket.java