Natura - nature
   actualia - actual entities
     Mundus Plinius - physical world
       naturalia - natural bodies
         "crystal-kingdom"
           chemical substances
             reactive intermediates
               Class diradicals [X:, X•Y•]
                 Class carbenes [X2C:, X=C:]
                   Class acyl carbenes [acyl-C:R]
                     Class carboxylic acyl carbenes [RC(=O)C:R]