@how rule upadezeja used to work anciently

Rule upadezej... literally says —

Vowels that are nasalized in an /upadeza are label-letters.

In ancient times, students where taught to chant the sUtra sasajuSoruH not like they chant it now, but instead like —

sa~sajuSoru~H सँसजुषोरुँः

making a noseleak at the first a~ अँ and at the last u~ उँ.

That is, all vowels that were label-letters were nasalized, to show that they were label-letters.

Nowadays the sUtras ar enot chanted like that, so you must ask someone which letters are labelletters.

Also in ancient times, the su~p सुँप् meaning "any /sup ending", and the sup सुप् meaning "the seventh plural ending /sup'", were pronounced differently —

rosu~pi रोसुँपि "replace ahan अहन् with ahar अहर् unless before any su~p सुँप् ending"

rossupi रोःसुपि, "r stays unchanged before the seventh plural ending sup सुप्"

But now they sound the same, to the annoyance of students.