Who repeats the prayer if neither the imam nor the ma'mum knows about the hadath?

Question : In summary, the stronger position is as follows:

Answer : If neither the imam nor the ma'mum knows about the hadath until the prayer is finished, who repeats it? The imam alone repeats it, while the ma'mum does not have to repeat it because he is excused for not knowing. If the imam realizes it during the prayer, he must leave and appoint someone to complete the prayer with them. If he does not appoint anyone, they appoint one of them. If they do not do that, they pray on their own, and they build upon their prayer, or do they not? Yes, they build upon their prayer; their prayer is valid.