code: func shiftBitmap(a *Container) (*Container, bool) {
  	statsHit("shift/Bitmap")
  	carry := false
  	output := NewContainerBitmap(a.n, nil)
  	ba, bo := a.bitmap(), output.bitmap()
  	lastCarry := false
  	for i, v := range ba {
  		carry = (v & (1 << 63)) != 0
  		v = v << 1
  		if lastCarry {
  			v |= 1
  		}
  		bo[i] = v
  		lastCarry = carry
  	}
  	if carry {
  		output.n--
  	}
  	return output, carry
  }
docs: // shiftBitmap is a bitmap-specific implementation of shift().