S/N values as a function of SYSREM iterations. We show the detection strengths obtained from cross-correlation with model spectra including the emission lines of ¹²CO, H₂O, and both species together, respectively. The iteration with the most significant S/N peak is indicated by the star symbol. In our retrieval framework in Sec. 5, we used the iteration number that yields the most prominent S/N detection peak of the species-combined signal, i.e., two consecutive SYSREM iterations.