Natura - nature
   actualia - actual entities
     Mundus Plinius - physical world
       naturalia - natural bodies
         "crystal-kingdom"
           chemical substances
             chemical compounds
               Class heteroarenes
                 pyran [C5H6O]
                   1 2H-pyran [C5H6O; ~OCH2[CH=CH]2~]
                   2 4H-pyran [C5H6O; ~OCH=CHCH2CH=CH~]