अल्लोपोऽनः

Operational rule vidhi

If a भसंज्ञक अङ्ग ends in the word 'अन्' then the 'अ' of this 'अन्' is omitted.

Word by word (padaccheda)

Substitute / term अत्- prathamā · ekavacanaTarget अनः ṣaṣṭhī · ekavacana

Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).

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

अत्- अनः

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

As a rewrite rule (sketch from case endings)

अनः → अत्-

Replace anaḥ with at-lopaḥ. Inherited words (anuvṛtti) may add further conditions.

Of a भ अङ्ग (stem) ending in अन् , the अ is elided.
As राज्ञः पश्य, राज्ञा, राज्ञे, तक्ष्णः पश्य, तक्ष्णा and तक्ष्णे ॥

Vart:- The elision takes place of a stem which actually ends in न्, therefore, it does not take place here, राजकीयम् ॥

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