/avagraha / अवग्रह

Whenever rule eGaHpadAnt... makes an a अ disappear, it is a good idea to write the /avagraha symbol (' ऽ) in the place where the a अ used to be:

**azvas अश्वस् + atra अत्र → **azvotra अश्वोत्र,
also spelled azvo 'tra अश्वो ऽत्र.

te ते + atra अत्र → tetra तेत्र,
also spelled te 'tra ते ऽत्र.

This is always done in careful printing nowadays (provided that the a अ is not inside a compound). Before the printing era, however, it was seldom used. And even nowadays most people use this symbol erratically. If at all.

Beginners should use this symbol wherever they can, but they should not trust that everybody else will do the same thing. So in cases like

kathayisyantitevyayam कथयिस्यन्तितेव्ययम्,

both vyayam व्ययम् and avyayam अव्ययम् must be suspected, regardless of the presence or absence of the symbol.