docs: public int nextSetBit(int index) {
          int i = index>>6;
          if (i>=wlen) return -1;
          int subIndex = index & 0x3f;      // index within the word
          long word = bits[i] >> subIndex;  // skip all the bits to the right of index
  
          if (word!=0) {
              return (i<<6) + subIndex + Long.numberOfTrailingZeros(word);
          }
  
          while(++i < wlen) {
              word = bits[i];
              if (word!=0) return (i<<6) + Long.numberOfTrailingZeros(word);
          }
  
          return -1;
      }