The long ā is substituted for the final a of a tense aṅga (stem) before a sārvadhātuka affix beginning with a v or m (literally a consonant of yañ pratyahāra).,
As पचामि, पचावः, पचामः, पक्ष्यामि, पक्ष्यावः, पक्ष्यामः ॥ Why 'for the अ only'? Observe चिनुवः, चिनुमः ॥ Why before a यञ् consonant (semivowels, nasals and झ and भ) only? Observe पचतः, पचथः ॥ Why a 'Sarvadhatuka'? Observe अङ्गना, केशवः ॥ Some read the anuvritti of तिङ् into this sutra, from 7.3.88 so that the lengthening takes place only before Personal-endings. According to them before क्वसु there is no lengthening, as भववान् ॥
The word भववान् is thus formed. To भू is added क्वसु, then comes शप्, treating kvasu as a sarvadhatuka under 3.4.117; the reduplication of the root ending in शप् is prevented, because the word dhatu is used in 6.1.8 which ordains reduplication of a root only, and not of a root plus a vikarana like the form भव (भू + शप्) ॥ Those who do not read the anuvritti of तिङ् in this sutra, but only of the word सार्वधातुके, they explain the form भववान् as a Vedic anomaly.,