शीर्षंश्छन्दसि

Operational rule vidhi

The word शीर्षन् 'head', is found in छन्दस्.

Word by word (padaccheda)

Substitute / term शीर्षन् prathamā · ekavacanaRight context छन्दसि saptamī · ekavacana

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

शीर्षन् छन्दसि

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

Commentary

The word शीर्षन् 'head', is found in छन्दस्।
This word is another form of शिरः and means 'head'. This is not a substitute of शिर in the Vedas, for both forms are found therein. Thus शीर्ष्णा हि तत्र सोमं क्रीतं हरन्ति, यत्ते शीर्ष्णो दौर्भाग्यम् ॥ In the secular literature there is only one form शिरः ॥

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