Histogram of the ALMA ¹³CO spectrum, convolved with a 5″ beam to increase the signal-to-noise ratio, along the line-of-sight to R Scl. The negative channels are due to significant missing flux near the front and back cap of the shell (see Appendix A), which does not affect the other channels with more compact emission. The dotted, short-dashed and long-dashed lines are three ¹³CO radiative transfer models (see Appendix B) with different values for the ¹²CO/¹³CO ratio only in the present-day mass loss. The ratio in the detached shell is kept at 19.