उपसर्गस्यायतौ

ल् is substituted for the र् of a preposition when अयते follows.
Thus पलायते, प्लायते ॥ Here arises the question, does the word अयति qualify the word र्, or does it qualify the word Preposition. In the first view, the sutra would mean, "the र् immediately followed by अयति is changed to ल ॥" But as a matter of fact, र् is never immediately followed by अयति ॥ Thus in प्र + अयति or परा + अयति, the letter अ and आ intervene respectively. The ekadesa sandhi of these, will make र् immediately followed by अयति; but the ekadesa, being sthanivat will prevent it. This difficulty however, is overcome by the maxim येन नाव्यवधानं तेन व्यवहितेऽपि वचनप्रामाण्यात्, for otherwise the rule will be useless. For the same reasons, परि + अयते = पल्ययते, though here र् intervenes between र् and अयते ॥ In short, the intervention of one letter is considered as no intervention.

In the second view, the sutra would mean when a Preposition is followed by ayat then its र् is changed to ल, and none of these difficulties will arise with regard to the above forms. But then would arise a fresh difficulty, for the र् of प्रति would also require to be changed into ल् ॥ Some say, that प्रति is never followed by अयति; while others hold that the form प्लत्ययते (प्रति + अयते) is valid. According to the first view, the valid form is प्रत्ययते ॥ The स् of the Prepositions दुस् and निस् is changed to र्, but this र् is not changed to ल्, because it is asiddha: thus we have the forms निरयणम्, दुरयणम् ॥ But there is a preposition निर् also the र् of which is changed to ल्, as निलयनम् ॥ See 7.2.46. According to the Siddhanta Kaumudi, there is a Preposition दुर् also, which gives दुलयते ॥