}
          remainingSetBits &= ~(1 << index);
          return elements.get(index);
        }
      };
    }

    @Override
    public int size() {
      return Integer.bitCount(mask);
    }

    @Override
    public boolean contains(@CheckForNull Object o) {
      Integer index = inputSet.get(o);
      return index != null && (mask & (1 << index)) != 0;
    }
  }