After a root ending in long ṝ , short u or long ū , there is the affix ap,
This debars घञ्. The प् is indicatory for the sake of accent 3.1.3; as, कृ + अप् = करः 'scattering'; गॄ--गरः 'beverage, poison'; शॄ--शरः 'arrow'; यु--यवः 'barley'; लू--लवः 'reaping'; पू--पवः 'winnowing corn'.
The द् in ॠद् is merely for the sake of euphony: it is not त् changed into द; for then by the rule 'that which follows त् is also called तपर' short उ only would have been taken and not long ऊ 1.1.70.,