उदक् च विपाशः

Operational rule vidhi

In the context of चतुरर्थाः, to indicate the name of a well by referring to a word having one or two स्वराः, अञ् प्रत्यय is used if the well exists in the northern region of the river विपाश् .

Word by word (padaccheda)

Substitute / term उदक् prathamā · ekavacanaIndeclinable च avyayaLeft context विपाशः pañcamī · ekavacana

Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
Indeclinable An avyaya such as च "and", वा "optionally", न "not".
Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.

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Full reading

उदक् च विपाशः

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

Commentary

The affix अञ् comes in the quadruple sense after any polysyllabic प्रातिपदिक (Nominal Stem) in denoting 'a well' situated on the Northern bank of the river विपाश।
This debars अण्; and it applies to all words whether polysyllabic or not. Thus दात्तः "a well completed by Datta", गौप्तः &c.

Why do we say on the "northern bank"? Because if a well is situate on the southern bank of the river Vipasa, then the affix अण् will apply as दात्तः and गौप्तः, the difference being in the accent. This sutra illustrates the refined distinctions drawn by the author, and the importance attached to accents in former times.

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