deriv LSK ETT STT aSTA ALPH OLDHOMEPAGE NEWHOMEPAGE
Sutra tasyalopaH say sthat labelletters disappear. How can I tell which letters are labelletters?
You can tell that z श् and p प् in the /zap and /tip are labelletters because sUtras halantyam ff teach so.
In the case of /su however, you are not so lucky. Nowadays, no sUtra teaches that the u उ in /su is a label but the u उ in /sup' is not a label. So you learn that this u उ of /su is a label when (A) teacher tells you so, or (B) you figure out by yourself that it must be a label because teacher said that **azva- अश्वॱ with /su makes **azvas अश्वस्.
Why do grammarians add labels to things?
Many reasons.
In sutra sasajuSoruH, the word sa स means s स् — this s स् has an a अ added after it just FEOP — namely, because sasajuSoruH ससजुषोरुः is easier to say than ssajuSoruH.
In sUtra kartarizap, the word zap शप् means a particular sort of a अ — it has z श् and p प् added so that you know, among other things, that you have to turn dru द्रु into dro द्रो before this particular a अ, because some sUtras teach the replacing of u उ with o ओ before affixes with p प् label.
In sUtra svamornapuMs..., that teaches the deleting of s स् after **madhu- मधुॱ, the s स् is called /su so that the sUtra does not delete every s स् that might come after **madhu- मधुॱ, but only the s स् called /su that was added because of GyApprAtipad... and svaujasamauTch.... That's why the s स् in **madhuSu मधुषु is not deleted.