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:]
                   1 Class alkylidenes [R2C:]
                   2 Class alkylidynes [RC:•]
                   3 Class acyl carbenes [acyl-C:R]
                   4 Class imidoyl carbenes [RC(=NR)C:R]
                   5 Class vinyl carbenes [R2C=CRC:R <-> R2C+CR=C-R]