commodities should be individuals
saref:Coal, saref:Electricity, etc. are defined as subclasses of saref:Commodity.
Although it may be useful to define categories of commodities by creating sub-classes of saref:Commodity, for example for s4abcd:AgriculturalCommodity or s4abcd:EnergyCommodity
It makes no sense to define an instance of saref:Coal or saref:Electricity. Instead, the ones defined in SAREF should be instances of the class saref:Commodity (and maybe s4abcd:EnergyCommodity)