2.3.72 तुल्यार्थैरतुलोपमाभ्यां तृतीयाऽन्यतरस्याम्

Word by word (padaccheda)

With / by तुल्य- अर्थैः tṛtīyā · bahuvacanaWith / by अ- तुला- उपमाभ्याम् tṛtīyā · dvivacanaSubstitute / term तृतीया prathamā · ekavacanaRight context अन्यतरस्याम् saptamī · ekavacana

With / by Tṛtīyā (3rd case): accompaniment or means.
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").

Full reading

तुल्य-अर्थैः अ-तुला-उपमाभ्याम् तृतीया अन्यतरस्याम्

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

Commentary

The 3rd or the 6th-Case-affix may optionally be employed, when the word is joined with another word meaning 'like to or resemblance', excepting तुला and उपना।
Thus तुल्यो देवदत्तेन or देवदत्तस्य, सदृशो देवदत्तेन, or देवदत्तस्य ॥ But with तुला and उपमा the Genitive must necessarily be employed. As तुला देवदत्तस्य नास्ति, उपमा कृष्णस्य न विद्यते ॥

Though the anuvritti of वा was understood in this sutra from the previous sutra, yet the repetition of अन्यतरस्याम् is for the sake of the subsequent sutra. The word च in 2.3.73 attracts the word अन्यतरस्याम् into that sutra; but had the word अन्यतरस्याम् not been used in this sutra, then the word च would have attracted the word तृतीया instead, which is not desired.

On this sutra, Professor Apte says:-- 'Panini says that the words तुला and उपमा can not be used with the Instrumental. But this is against good usage : as तुलां, यदारोहति दंतवाससा (Kum. Sam. V. 24), नभसा तुलां समाररोह (Raghuvamsa VIII.15). स्फुठोपमं भूतिसितेन शंभुना (Magh I.4).'

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