In the chandas (Vedas) sasūva is irregularly formed in the liṭ (Perfect Tense).,
It is derived from सू ॥ Thus ससूव स्थविरं विपश्चिताम् otherwise सुषुवे ॥ Rig. IV. 18 10.
This word ससूव might well have been included in sutra 7.4.65.,
ससूवेति निगमे
Adhyāya 7 · Pāda 4 · Rule 74