Natyashastra
वृत्तौ निवृत्तयोगश्च गते वस्तुचतुष्टये । शाखायां प्रतिशाखायां विशेषः पश्चिमे त्वयम् ॥ २४३॥
In the Vṛtta, the combination with Nivṛtta is applied when the four Vastus are completed. In the Śākhā and Pratiśākhā, there is a special rule for the western half.
english translation
vRttau nivRttayogazca gate vastucatuSTaye । zAkhAyAM pratizAkhAyAM vizeSaH pazcime tvayam ॥ 243॥
hk transliteration by Sanscript