अत्वसन्तस्य चाधातोः

In the Nominative 1st-Case singular (with the exception of the Vocative Case singular) the penultimate vowel is lengthened in a अङ्ग (stem) ending in अतु and अस् when the consonant (अस्) does not belong to a root.
Thus भवान् (with डवतुप् Unadi 1.63 from भा to shine), कृतवान् (with क्तवतु); गोमान् and यवमान् (with मतुप्) ॥ The नुम् (न्) is added to the above by 7.1.70, after the elongation has taken place, for if added before elongation, the vowel no longer being penultimate, will not be lengthened at all. अस् :-as सुपयाः, सुयशाः, सुश्रोताः ॥ Why do we say 'not belonging to a dhatu'? Observe पिण्डग्रः where स् belongs to the root ग्रस् (पिण्डं ग्रसते), so also चर्मवः (चर्म वस्ते)॥ The अस् having no significance as an affix &c is also included here, on the strength of the maxim "whenever अम् or इन् or अस् or मम्, when they are taught in Grammar denote by 1.1.72, something that ends with these, there they represent these combinations of letters both in so far as they possess, and also in so far as they are void of a meaning". (अनिनस्मन् ग्रहानि अर्थवता चानर्थकेन च तदन्तविधिं प्रयोजयन्ति) ॥ The word अन्त in the sutra indicates whatever ends in अतु whether when first enunciated (उपदेश), such as डवतु, क्तवतु &c, or which assumes the form अतु in grammatical inflection, such as मतुप् which in upadesa ends in अतुप्, but becomes अतु in प्रयोग or application. In Vocative singular we have हे गोमन्, हे सुपयः this rule not applying there.