Date: Purim 5573
From: Rabbi Moshe Sofer
To: Rabbi Simcha Bunim Ginz (Eiger)
The Pri Megadim asks: if 'af hen hayu be'oso ha-nes' obligates women in Megillah and Chanukah, why doesn't the Exodus reference in tefillin, tzitzis, and kiddush similarly obligate them? The Chasam Sofer distinguishes between mitzvos whose entire essence commemorates the miracle versus those where the Exodus is merely a subsidiary mention.
Source: Shu"t Chasam Sofer OC §185