King Shapur -- The correct explanation appears to accord with Rashi (that King Shapur here refers to the actual King Shapur, not the sage Shemuel who is sometimes called King Shapur), for King Shapur was an expert in Jewish law, as it says at the end of Tractate Avodah Zarah (76b), that he stabbed the knife ten times into hard earth when he cut the citron with a knife for Rav Yehudah.