In a mantra the finals of the preceding member ending in i or u are lengthened before the particle su,
Thus अभी षु णः सखीनाम् (Rig. IV. 31. 3) उर्ध्व ऊ षुण उतये (Rig I. 36. 13). The स is changed to ष by 8.3.107: and न changed to ण by 8.4.27.,
इकः सुञि
Adhyāya 6 · Pāda 3 · Rule 134