रुहः पोऽन्यतरस्याम्

Operational rule vidhi

प् may optionally be substituted for the final of रुह् 'to mount' in the Causative.

Word by word (padaccheda)

Target रुहः ṣaṣṭhī · ekavacanaSubstitute / term पः prathamā · ekavacanaRight context अन्यतरस्याम् saptamī · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
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

रुहः पः अन्यतरस्याम्

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)

रुहः → पः / अन्यतरस्याम्

Replace ruhaḥ with paḥ when anyatarasyām follows. Inherited words (anuvṛtti) may add further conditions.

Commentary

प् may optionally be substituted for the final of रुह् 'to mount' in the Causative.
As व्रीहीन् रोपयति or रोहयति ॥ According to Padamanjari this sutra could be dispensed with. The form रोपयति could be obtained from the root रुप् of Divadi class : which though meaning मोहन may be taken to mean 'grow' also; अनेकार्थत्वाद् धातूनाम् ॥

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