What is added to Birkat HaMazon on Shabbat, Yom Tov, Chanukah, and Purim? Does the Rambam just add an additional prayer on Shabbat and Yom Tov, or does he actually change the text of the beracha? Why does the Rambam seem to rule neither like the Talmud Bavli nor like the Talmud Yerushalmi regarding Al HaNisim?