प्रष्ठोऽग्रगामिनि

Operational rule vidhi

The word प्रष्ठ is irregularly formed when denoting 'a chief' or 'one who goes in front'.

Word by word (padaccheda)

Substitute / term प्रष्ठः prathamā · ekavacanaRight context अग्रगामिनि 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

प्रष्ठः अग्रगामिनि

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

Commentary

The word प्रष्ठ is irregularly formed when denoting 'a chief' or 'one who goes in front'.
Thus प्रष्ठोऽश्व meaning a 'best horse' Why do we say when meaning 'going in front'? Observe प्रस्थे हिमवतः पुण्ये "On the sacred peak of the Himalaya". प्रस्थो व्रीहीणाम् 'a measure of barley'.

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