public void clear(long index) {
        assert index >= 0 && index < (wlen << 6);

        int wordNum = (int) (index >> 6); // div 64
        if (wordNum >= wlen) return;
        int bit = (int) index & 0x3f; // mod 64
        long bitmask = 1L << bit;
        setWord(wordNum, word(wordNum) & (~bitmask));
    }
clears a bit

@param index the index to clear
indexr-common/src/main/java/io/indexr/util/DirectBitMap.java