The na of kṛt affix ordained after a causative verb (ṇayanta), is optionally changed into ṇa , when it comes after an upasarga having a cause of change.,
Thus प्रयापणम् or प्रयापनम्, परियापणम् or परियापनम्, प्रयाप्यमाणम् or प्रयाप्यमानम्, प्रयापणीयम् or प्रयापनीयं, अप्रयापणिः or अप्रयापनिः, प्रयापिणौ or प्रयापिनौ ॥
Why have we used the word विहितं 'ordained' in explaining the sutra? Because the change takes place, even where the कृत् affix does not follow directly after the affix णि, where a third affix such as यक् intervenes. As प्रयाप्यमाणम् ॥ The णेः being in the ablative, under the ordinary rule of interpretation 1.1.72, the krit-affix should come immediately after णि.,