Before any other Present character (śit) the root vowel of 1. ṣṭhiv 2. klam and 3.ā-cam is lengthened.,
As ष्ठीवति, क्लामति, and आ चामति ॥ क्लम् lengthens its vowel before श्यन् by the last rule, the present rule produces this change before the affix शप् also, which it gets by 3.1.70. चम् lengthens its vowel only when it is preceded by आ; therefore not here: चमति, विचमति or उचमति ॥ The sutra is exhibited as ष्ठिवुक्लमु चमां शिति in the original text of Panini; the present form, owes its existence to the insertion of आ from the vartika दीर्घत्वमाङि चमः ॥,