स्वपिस्यमिव्येञां यङि

Operational rule vidhi

There is vocalisation of the semi-vowel, of the verbs स्वप् 'to sleep', स्यम् 'to shout' and व्ये 'to cover', when followed by the affix यङ् of the Intensive.

Word by word (padaccheda)

Target स्वपि- स्यमि- व्येञाम् ṣaṣṭhī · bahuvacanaRight context यङि saptamī · ekavacana

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

स्वपि-स्यमि-व्येञाम् यङि

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

Commentary

There is vocalisation of the semi-vowel, of the verbs स्वप् 'to sleep', स्यम् 'to shout' and व्ये 'to cover', when followed by the affix यङ् of the Intensive.
Thus सोषुप्यते, सेसिम्यते, वेवीयते 7.4.25 the short इ is lengthened). Why do we say 'when यङ् follows'? Observe स्वप्नक् formed by नजिङ् 3.2.172.

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