The source of the agent of the verb bhū 'to become' is called apādāna,
The phrase 'of the agent (kartuh)' is understood here. The word प्रभव means the source or that from which anything arises. As हिमवतो गंगा प्रभवति the Ganges has its source in the Himalayas. काश्मीरेभ्यो वितस्ता प्रभवति. The Hydaspes has its source in Cashmere.,