For cāy is diversely substituted kī in the chandas,
Thus विधुना निचिक्युः, नान्यं चिक्युर्न निचिक्युरन्यम् ॥ These are forms ending in the affix उस् of the Perfect. Sometimes there is no substitution. As अग्निर्ज्योतिर्निचाय्यः ॥,
चायः की
Adhyāya 6 · Pāda 1 · Rule 35