[Chebi-ontology] Oxygen-18 to isotope: instance or subclass?

[Barry S. <phi...@bu...>]

Re: the issue raised by:

​    oxygen-18
*        subclass of* oxygen
        *instance of* isotope​

​can I ask that people help me get clear about the background:

(a) is it the case that all of these expressions refer to general entities
which have many many particular instances

(b) is it the case ​that every instance of oxygen-18 is also an instance of
isotope

​(c) if yes, then why not just assert 'subclass of' throughout?

BS​