In yu and plu , long is substituted for u in the chandas (Vedas).,
Thus दान्त्यनुपूर्वं वियूय; यत्रा यो दक्षिणा परिप्लूय ॥ Why do we say 'in the Chhandas'? Observe संपुत्य, आप्लुत्य in the secular literature.,
यु-प्लुवोर् दीर्घश् छन्दसि
Adhyāya 6 · Pāda 4 · Rule 58