In a compound, the second member of which is a monosyllable, there is ṇa in the room of na of the second member, provided that the na is at the end of a prātipadika , or is the augment num , or occurs in a vibhakti; and when the first member contains a cause of change.,
Thus वृत्रहणौ and वृत्रहणः 'the Vritra-killer'. क्षीरपाणि 'Drinkers of milk' : सुरापाणि 'drinkers of wine' : being examples of नुम् 7.3.88, क्षीरापेण and सुरापेण are examples of case-terminations.
Why the letter ण is repeated in this sutra, while its anuvritti was understood from the context? It is repeated in order to show that this is not an optional rule, but an obligatory rule. In fact, it shows that the anuvritti of वा 'optional' ceases, and does not extend further.,