code: func (ba *bitArray) ClearBit(k uint64) error {
  	if k >= ba.Capacity() {
  		return OutOfRangeError(k)
  	}
  
  	if !ba.anyset { // nothing is set, might as well bail
  		return nil
  	}
  
  	i, pos := getIndexAndRemainder(k)
  	ba.blocks[i] &^= block(1 << pos)
  
  	if k == ba.highest {
  		ba.setHighest()
  	} else if k == ba.lowest {
  		ba.setLowest()
  	}
  	return nil
  }
docs: //ClearBit will unset a bit at the given index if it is set.