The substitution of for s takes place then also, when the augment num (n) the visarjanīya or a sibilant occurs between the said iṇ and ku letters or the s,
The word व्यवाय 'separation, intervention' applies to every one of the words नुम्, &c. Thus (1) when नुम् intervenes, as सर्पींषि, यजूंषि, हवींषि 7.1.72, 6.4.10. (2) When a visarjaniya intervenes, as सर्पिःषु, यजुःषु, हविःषु 8.3.36 (3) When a Sibilant intervenes, as सर्पिष्षु, यजुष्षु हविष्षु ॥
The षत्व takes place, when नुम् &c intervene singly and not when they intervene collectively. Therefore not here, निंस्से, निंस्स्वे from the root निंस् 'to kiss'. Here there is the intervention of two, namely, नुम् and स् 3.4.91.
The word इण्कोः is in the Ablative case, and it required that the स् should follow immediately after it. Hence the necessity of the present sutra for the intervention of certain letters.,