@SuppressWarnings("unchecked")
    public K key(int i) {
        return (K) store[i];
    }
Obtain a key. Keys are accessed in the order they were first
inserted.

@return the requested key
@param i the index of the key, must be in the range [0, size() - 1]
vespajlib/src/main/java/com/yahoo/collections/Hashlet.java