हैहेप्रयोगे हैहयोः

Operational rule vidhi

If the words हे / है are used in a sentence that is used for calling someone from a distance, then the स्वर of the only हे / है becomes प्लुत and उदात्त.

Word by word (padaccheda)

Right context हैहेप्रयोगे saptamī · ekavacanaTarget हैहयोः ṣaṣṭhī · dvivacana

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

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Full reading

हैहेप्रयोगे हैहयोः

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

Commentary

When the words है and हे are used in addressing a person from a distance, there the है and हे alone get the प्लुत and the accent.
As है ३ देवदत्त, देवदत्त है ३ ॥ हे ३ देवदत्त, देवदत्त हे ३ ॥

The repetition of the words है and हे in the sutra, is for the sake of indicating, that the prolation takes place even when है and हे do not stand at the end of a sentence.

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