कृच्छ्रगहनयोः कषः

Operational rule vidhi

The participial affix does not take the augment इट् after the root कष् when the participle means 'difficult' and 'impenetrable'.

Word by word (padaccheda)

Right context कृच्छ्रगहनयोः saptamī · dvivacanaTarget कषः ṣaṣṭhī · ekavacana

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

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Full reading

कृच्छ्रगहनयोः कषः

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

Commentary

The participial affix does not take the augment इट् after the root कष् when the participle means 'difficult' and 'impenetrable'.
As कष्टोऽग्निः, कष्टं व्याकरणं, ततोऽपि कष्टतराणि सामानि ॥ "Difficult is Fire-sacrifice i. e. it is difficult to completely master the ritual connected with the worship of fire; and difficult enough is Grammar, but the Samans are worst of all". कष्टानि वनानि 'impervious forests'. कष्टाः पर्वताः ॥ When not having these senses, we have कषितं सुवर्णम् ॥

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