code: func (c *Container) bitmapXorRange(i, j uint64) {
  	x := i >> 6
  	y := (j - 1) >> 6
  	var X uint64 = maxBitmap << (i % 64)
  	var Y uint64 = maxBitmap >> (63 - ((j - 1) % 64))
  	bitmap := c.bitmap()
  	if x == y {
  		cnt := popcount(bitmap[x])
  		bitmap[x] ^= (X & Y) //// flip
  		c.n += int32(popcount(bitmap[x]) - cnt)
  	} else {
  		cnt := popcount(bitmap[x])
  		bitmap[x] ^= X
  		c.n += int32(popcount(bitmap[x]) - cnt)
  		for i := x + 1; i < y; i++ {
  			cnt = popcount(bitmap[i])
  			bitmap[i] ^= maxBitmap
  			c.n += int32(popcount(bitmap[i]) - cnt)
  		}
  		cnt = popcount(bitmap[y])
  		bitmap[y] ^= Y
  		c.n += int32(popcount(bitmap[y]) - cnt)
  	}
  }
docs: // xor's all bits in [i, j) with all true (c must be a bitmap container).