The affix ap comes after the verb hve 'to call' and there is the change of the semi-vowel to its corresponding vowel, when the root is without an upasarga and mere action is denoted.,
As हवः 'calling'; हवे हवे सुहवं शूरमिन्द्रम् ।
Why do we say 'when no upasarga is in composition with it'? Observe आह्वायः । Though the anuvritti of the word भाव was understood in this aphorism from sutra 18, its repetition here shows that the anuvritti of sutra 19. which was running, must not be taken; so that this word applies to agent-karaka also.,