The seṭ ktvā is optionally kit after the verbs vañc 'to cheat', luñc 'to pluck' and ṛt 'to dare or abhor'.,
After the above three verbs, the ktva affix being set, may or may not be kit. As वचित्वा or वंचित्वा 'having cheated.' So also लुचित्वा or लुंचित्वा 'having plucked', ऋतित्वा or अर्तित्वा 'having dared.'
When the verb ऋत् rit takes the affix ईयङ् it is not governed by this sutra. See sutra 3.1.29.
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When the ktva is not set, but anit, then there is no option allowed. As वच् + क्त्वा = वक्त्वा.,