ru is substituted for the n of svatavān before pāyu,
As स्वतवाँ पा॒॒युर॑ग्ने॒॒ (Rig. IV. 2. 6). The word is स्वतवस्, the नुम् is added by (VII. I. 83). The word is derived from तु वृद्धौ with the affix असुन् (स्वन्तवो यस्यऽसौ स्वतवान्) ॥,
स्वतवान् पायौ
Adhyāya 8 · Pāda 3 · Rule 11