[chemical substances]
   compounds
     Class onium compounds
       1.1.1 Class ammonium compounds [R4N+X-]
       1.1.2 Class iminium compounds [R2C=NR2]+X-
       1.2 Class phosphonium compounds [[R4P]+X-]
       1.3 Class arsonium compounds [[R4As]+X-]
       1.4 Class stibonium compounds [[R4Sb]+X-]
       2 Class sulfonium compounds [[R3S]+X-]