code: @Override
  	public BitVector bitVector() {
  		final LongArrayBitVector result = LongArrayBitVector.getInstance().length(n);
  		long prev = 0, rank;
  		for(long i = 1; i <= n; i++) {
  			if ((rank = rank(i)) != prev) result.set(i - 1);
  			prev = rank;
  		}
  		return result;
  	}
docs: Returns the bit vector indexed; since the bits are not stored in this data structure,
  a copy is built on purpose and returned.
  
  <p><strong>Warning</strong>: this method is very slow, as the only way to recover the original bit
  array is to rank each position in the bit vector.
  
  @return a copy of the underlying bit vector.