[chemical substances]
   compounds
     Class carbohydrates [Cn(H2O)n, etc.]
       Class saccharides
         Class monosaccharides - sugars
           Class ketoses [H[CHOH]nC(=O)[CHOH]mH, etc.]
             1 [fructose] [C6H12O6; HOCH2C(=O)[CHOH]3CH2OH] - fruit sugar