The kṛt affix, coming after a verb, which begins with a consonant and has a penultimate ic vowel, changes optionally its n , which is preceded by a vowel, into ṇ , when it comes after upasarga having a cause of change.,
The phrase कृत्यचः is understood here. Thus प्रकोपणम् or प्रकोपनम्, परिकोपणम् or परिकोपनम् ॥
Why do we say हलः 'beginning with a consonant'? Observe प्रेहणम् and प्रोहणम् where the change is obligatory, under 8.4.29. These are from ईह चेष्टायाम् and ऊह वितर्के ॥
Why do we say इजुपधात् 'having a penultimate इच् vowel'? Observe प्रवपणम् and परिवपणम्; no option.
By the rule कृत्यचः the change was obligatory, this rule makes it optional.
The न of the krit-affix must come after a vowel, otherwise there will be no change. See परिभुग्नः ॥
The word हलः in the sutra should be interpreted as हलादेः, and not as ordinarily \after what ends in हल्\ ॥ For an इजुपधा verb implies that it ends with a consonant, and so the employment of the word हलः in the text would have been useless had it meant हलन्तात् ॥,