docs: public void fastClear(int index) {
          assert index >= 0 && index < numBits;
          int wordNum = index >> 6;
          int bit = index & 0x03f;
          long bitmask = 1L << bit;
          bits[wordNum] &= ~bitmask;
          // hmmm, it takes one more instruction to clear than it does to set... any
          // way to work around this?  If there were only 63 bits per word, we could
          // use a right shift of 10111111...111 in binary to position the 0 in the
          // correct place (using sign extension).
          // Could also use Long.rotateRight() or rotateLeft() *if* they were converted
          // by the JVM into a native instruction.
          // bits[word] &= Long.rotateLeft(0xfffffffe,bit);
      }