Natura - nature
   actualia - actual entities
     Mundus Plinius - physical world
       naturalia - natural bodies
         "crystal-kingdom"
           chemical substances
             reactive intermediates
               Class cations [X+]
                 Class onium compounds
                   Class halonium ions [R2X+ (X=halogen)]
                     1 Class fluoronium ions [R2F+]
                     2 Class chloronium ions [R2Cl+]
                     3 Class bromonium ions [R2Br+]
                     4 Class iodonium ions [R2I+]