Natura - nature
   actualia - actual entities
     Mundus Plinius - physical world
       naturalia - natural bodies
         "crystal-kingdom"
           chemical substances
             chemical compounds
               Class amides [RkE(=O)l(OH)m (l≠0; ≥1 (OH):=(NR2))]
                 Class primary amides [acyl-NR2]
                   Class carboxamides [RC(=O)NR2]
                     melatonin [C13H16N2O2]