public static long makeNthBucketKey(long n, int distributionBits) {
        return (n << (64 - distributionBits)) | distributionBits;
    }
Directly generate a bucket Id key for the <code>n</code>th bucket in
reverse sorted order.

@param n a number in the range [0, 2**<code>distributionBits</code>)
@param distributionBits Distribution bit count for the generated key
@return A value where, if you had generated 2**<code>distributionBits</code>
{@link BucketId}s with incremental numerical IDs and then sorted
them on their reverse bit-order keys, the returned key would be equal
to the nth element in the resulting sorted sequence.
documentapi/src/main/java/com/yahoo/documentapi/ProgressToken.java