The na , when preceded by a , in the root han , is changed to ṇa , when the verb is preceded by a preposition competent to cause the change.,
Thus प्रहण्यते, परिहण्यते, प्रहणनम्म् परिहणनम् ॥
Why do we say अत्पूर्वस्य 'when preceded by अ'? When अ is elided, the change does not take place. As प्रघ्नन्ति, परिघ्नन्ति 6.4.98, and 8.3.54.
Why do we say अत् \preceded by short अ\? Observe पर्यघानि and प्रघानि, the Aorist of हन् by the affix चिण् 3.1.60 प्रहन् + चिण् = प्रघन् + चिण् 8.3.54 = प्रघान् + चिण् 8.2.116 = प्रघानि 'he killed'. Here न being preceded by the long आ is not changed.,