From: Rabbi Moshe Sofer
To: Unknown recipient
This is an independent scholarly essay rather than a response to a specific question. The Chasam Sofer surveys four classical Rishonic approaches to deriving the Torah source for a niddah's obligation to immerse before resuming marital relations — the Bahag's kal vachomer, Rabbeinu Tam's derivation from 'mei niddah,' the Ri's derivation from 'v'hadavah b'niddassah,' and the Rambam's binyan av — and adds original insights showing how each position coheres with a broader system of halachic opinions.
Source: Shu"t Chasam Sofer YD §194