Rav Shimon ben Yitzchak Elchanan HaGadol of Mainz, Germany was a tenth century rabbi and scholar, a member of the school that eventually gave rise to Rashi and to the Tosafists of France and Germany. He composed "O King, validate" which can be found on page 268 in the Artscroll Rosh Hashanah Machzor. His son Elchanan was kidnapped as a child and eventually became Antipope Anacletus II. (Matzreif Dahava; see ArtScroll Rishonim, p. 118)