Natural kinds are expressly not to be classified as 'compound'. The class 'compound' is only to be used to classify amounts of matter, with non-semantic auxiliaries used to refer to chemical composition. The semantics of natural kinds such as "helium", "steel", or "hydrocarbon" is out of scope for IDO.
A typical RDL includes classes that categorise individuals according to material composition. This class is suitable as the top node of such a taxonomy.
Renamed class. Checked materially in Oxford English Dictionary.
