The affix yuc comes in the chandas (Vedas) after the roots having the sense of 'to go' when the word īṣad etc. meaning 'lightly' or 'with difficulty' are in composition with such verbs.,
This debars the affix खल्. Thus सूपसदनोऽग्निः । सूपसदनमन्तरिक्षम् । See T.S. 7. 5. 20. I,