पूर्वो ऽभ्यासः

The word द्वे in the nominative case is understood here, but for the purposes of this sutra it is taken in the Genitive case i. e. द्वयोर्यः पूर्वः \the first of the two\ ordained above. The word Abhyasa occurs in sutras 3.1.6, 6.1.7 &c.,