The augment num is added to labh 'to get' preceded by a Preposition before the affixes khal -- īṣadduḥsuṣu kṛcchrākṛcchrārtheṣu khal [[3.3.126]], and ghañ when su or duḥ alone (without another Preposition along with them) precede the root.,
Thus सुलभम्, दुर्लभम्, सुलाभो, दुर्लाभः ॥ But सुप्रलम्भः, दुष्प्रलम्भः ॥ The word केवल is used in the sutra because सुदुर्भ्यो is in the Instrumental case and not in the Ablative. Had it been in the Ablative, there would have been no necessity of using the word केवलं, for the rule would not have applied, when a preposition intervened between these and the root. In अतिसुलभम्, the word अति is not an Upasarga but a Karmapravachaniya; when अति is used as an Upasarga, we have अतिसुलम्भः ॥ If the words सुदुर्भ्याम् be construed as Ablative, then also the use of केवल is for a purpose similar to that as above.,