[chemical substances]
   compounds
     Class hydrocarbons [CxHy] - compounds consisting of carbon and hydrogen only
       Class olefins
         1.1 Class alkenes [CnH2n] - unsaturated acyclic hydrocarbons having one double bond
         1.2 Class alkadienes - unsaturated acyclic hydrocarbons having two double bonds
         1.3 Class alkatrienes - unsaturated acyclic hydrocarbons having three double bonds
         2.1 Class cycloalkenes
         2.2 Class cycloalkadienes
         2.3 Class cycloalkatrienes