उत्तमैकाभ्यां च

Operational rule vidhi

And also after the word that stands last in the above list (i.e. पुण्य) and एक this substitution does not take place.

Word by word (padaccheda)

Left context उत्तमैकाभ्याम् pañcamī · dvivacanaIndeclinable च avyaya

Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
Indeclinable An avyaya such as च "and", वा "optionally", न "not".

Dotted words are technical terms. Hover or tap one to see its card.

Full reading

उत्तमैकाभ्याम् च

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

Commentary

And also after the word that stands last in the above list (i.e. पुण्य) and एक this substitution does not take place.
The word उत्तम means 'last'; and refers to पुण्य which is the last word in 5.4.87. The word उत्तम 'last' is used instead of पुण्य as a वैचित्र ॥ Thus पुण्याहः, एकाहः ॥ Some include the penultimate word संख्यात also in the prohibition. According to them संख्याताहः is the proper form and not संख्याताह्नः ॥

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.