विभाषा पुरुषे

Operational rule vidhi

का is optionally the substitute of कु when the word पुरुष follows.

Word by word (padaccheda)

Substitute / term prathamā · ekavacanaRight context पुरुषे saptamī · ekavacana

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

पुरुषे

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

Commentary

का is optionally the substitute of कु when the word पुरुष follows.
Thus कापुरुषः or कुपुरुषः ॥ This is an aprapta-vibhasha. In the sense of 'a little ईषद्', the substitution is compulsory; as ईषत् पुरुषः = का पुरुषः ॥

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