/zreyas- / श्रेयस्

The base !**zreyas- mfn "better" is formed with the affix yas (Iyasun), which is /udit.

Being /udit, it gets /num by ugidacAMs... before all strong, and lengthens by sAntamahat... before all strong except the /sambuddhi, like this —

**zreyas- + /am
→ zreyans- + /am by ugidacAMs...
→ zreyAns- + /am by sAntamahat...
→ !**zreyAMsam " o better" by monusvAraH

**zreyas- + /su
→ zreyanss by ugidacAMs...
→ zreyAnss by sAntamahat...
→ zreyAns by halGyAbbhyodIrgh..., so now the other s is padafinal
→ zreyAn by saMyogAntasy...
→ !**zreyAn " s better"

If the /su is the /sambuddhi, there is no lengthening —

**zreyas- + /sambuddhi
→ zreyanss
→ zreyans
→ !**zreyan