यजुष्येकेषाम्

Operational rule vidhi

In the opinion of some, the above change takes place in the यजुर्वेद also.

Word by word (padaccheda)

Right context यजुषि saptamī · ekavacanaTarget एकेषाम् ṣaṣṭhī · bahuvacana

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).

Full reading

यजुषि एकेषाम्

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

Commentary

In the opinion of some, the above change takes place in the यजुर्वेद also.
Thus अर्चिर्भिष्ट्वम् or अर्चिर्भिस्त्वम् । अग्निष्टेग्रम् or अग्निस्तेग्रम् । अग्निष्टत् or अग्निस्तत् । अर्चिर्भिष्टतक्षुः or अर्चिर्भिस्ततक्षुः ॥

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