docs: private static int[] bitSetToIntVector(BitSet ints) {
          int intCount = ints.cardinality();
          if (intCount == 0) {
              return null;
          }
          int[] result = new int[intCount];
          int nextBit = ints.nextSetBit(0);
          for (int ii = 0; ii < intCount; ii++) {
              assert(nextBit != -1);
              result[ii] = nextBit;
              nextBit = ints.nextSetBit(nextBit+1);
          }
          assert(nextBit == -1);
          return result;
      }