गमेरिट् परस्मैपदेषु

An आर्धधातुक affix beginning with स् gets the इट् augment after गम् 'to go' in the परस्मैपद ।
As गमिष्यति, अगमिष्यत्, जिगमिषति ॥ Why of गम् ? Observe चेष्यति ॥ The repetition of इट् shows that the rule is invariable. Why 'in the Parasmaipada'? Observe संगंसीष्ट, संगसीष्ट, संगंस्यते, संजिगंसते, संजिगंसिष्यते, अधिजिगांसते, अधिजिगांसिष्यते ॥ The lengthening takes place by 6.4.16 when गम् is the substitute of the root इङ् 2.4.48 Why before स् ? Observe गन्तास्मि, गन्तास्वः, गन्तास्मः ॥

Ishti: This इट् augment is not desired of the root गम् standing in the same pada with an atmanepada affix. But it occurs every where else. As जिगमिषिता इव आचरति = जिगामिंषित्रीयते, here there is इट् augment, because atmanepada affix is not in the same pada with गम्, but is bahiranga. Compare 7.2.36 vartika. It occurs before krit affixes, and even where is luk-elsion of Parasmaipada affix, and where therefore the affix generally would have left no trace behind by 1.1.63. As संजिगमिषिता and अधिजिगमिषिता व्याकरणस्य ॥ So also जिगमिष त्वम्, here there is luk-elision of the Imperative affix हि ॥ The opinion of the author of Padaseshakara is that the employment of the term Parasmaipada in the sutra is illustrative. He explains it by saying:- परस्मैपदेषु यो गमिरुपलक्षितस्तस्मात् सकारादेरार्धधातुकस्य इट् भवति ॥ According to him we have the forms संजिगंसिता and अधिजिगंसिता व्याकरणस्य ॥