code: func (c *Container) countRange(start, end int32) (n int32) {
  	if c.isArray() {
  		return c.arrayCountRange(start, end)
  	} else if c.isRun() {
  		return c.runCountRange(start, end)
  	}
  	return c.bitmapCountRange(start, end)
  }
docs: // countRange counts the number of bits set between [start, end).