नित्यं संज्ञाछन्दसोः

Operational rule vidhi

The affix ङीप् necessarily comes in the छन्दस् (Vedas) and in forming Names after that बहुव्रीहि समास which ending in the syllable अन् loses its penultimate अ.

Word by word (padaccheda)

Substitute / term नित्यम् prathamā · ekavacanaRight context संज्ञा- छन्दसोः saptamī · dvivacana

Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").

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Full reading

नित्यम् संज्ञा-छन्दसोः

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

Commentary

The affix ङीप् necessarily comes in the छन्दस् (Vedas) and in forming Names after that बहुव्रीहि समास which ending in the syllable अन् loses its penultimate अ।
This is an exception to the last rule; for while that enjoined the optional, the present enjoins the compulsory application of the affix ङीप् ॥ Thus सुराज्ञी 'the city called Surajni'; अतिराज्ञी 'the city of Atirajni'. Similarly in the Chhandas we have गौः पञ्चदाम्नी, एकदाम्नी, द्विदाम्नी 'a cow having five udders, one udder, or three udders'. एकमूर्ध्रा समानमूर्ध्री ॥

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