चोः कुः

A Guttural is substituted for a palatal before a झल् (all consonant except semi-vowels and nasals) affix or at the end of a word.
Thus पक्ता, पक्तुम्, पक्तव्यम् and ओदनपक् from पच् ॥ Similarly वक्ता, वक्तम्, वक्तव्यम् and वाक् ॥

In क्रुञ्चा the feminine in टाप् of क्रुञ्च (क्रुङ्) by 4.1.4 list, the ञ् a palatal is followed by च् a jhal letter, and therefore, it should be changed to a guttural. It is not so, because Panini himself uses this word, in this form, in sutra 3.2.59. Or because the rule is confined to सङ् affixes only. Or the root is कुञ्च् without र् and with a penultimate न्, and not ञ् as we find in Dhatupatha कुन्च कौटिल्याल्पी भावयोः (Bhuadi 200). With the elision of न we have निकुचितिः before the क्तिन् affix 6.4.24 कुचितः in Past Participle, and अचोकुन् ॥ In निकुचितम् we cannot have the optional कित् of the Nishtha by 1.2.21, because the elision of the penultimate न् by 6.4.24 preceded on the basis of the affix being कित्, thus कुन्च् + क्त = कुच् + त ॥ This elision of न्, will not make the root उदुपधा for the purposes of the application of rule 1.2.21, on the maxim सन्निपातलक्षणो विधिरनिमित्तं तद्विघातस्य ॥ In fact, one of the reasons on which this maxim is based, is this very fact, that the elision of न् does not make the root उपधा for the purposes of making the affix non-कित् ॥ The affix क्तिन् takes the augment इट् under 7.2.9 (vartika). The word क्रुङ् is formed from this root by क्विन् affix 3.2.59: the final च् is first elided by 8.2.23, and then ञ् is changed to ङ् by 8.2.62. The rule 6.4.24 thus finds no scope here.

In this view of the case we say क्रुञ्च् is an irregular form of this root kunch, because it is so exhibited in 3.2.59. There the anusvara and parasavarna change of this न् to ञ् by 8.3.24 being considered asiddha, there is no palatal ञ्, and hence there is no guttural change.