An unknown carbohydrate of formula C12H22O11 reacts with Tollens reagent to form a silver mirror. An a glycosidase has no effect on the carbohydrate, but ß-glycosidase hydrolyzes it to D-galactose and D-mannose. When the carbohydrate is methylated (using methyl iodide and silver oxide) and then hydrolyzed with dilute HCl, the products are 2,3,4,6-tetra-O-methylgalactose and 2,3,4-tri-O-methylmannose. Propose a structure for this unknown carbohydrate.