The affix ap comes after the verb hve 'to call' when the preposition ni , abhi , upa and vi are in composition with it and its semi-vowel is changed into the corresponding vowel.,
This debars घञ्. As नि + ह्वे + अप् = नि + हु + अ 6.1.108 = नि + हो + अ 7.3.84 = निहव 6.1.78 'invocation'; अभिहवः 'invocation'; उपहवः 'invitation'; विहवः 'calling'. But प्रह्वायः 'a summons'.,