To clarify, a conceptset expression, as created in ATLAS and generated from webAPI, only relies on CONCEPT_ANCESTOR to expand out into an ‘included concepts’ list. It does not, and never did, use the CONCEPT_RELATIONSHIP for this purpose. To create the ‘included source concepts’, we must use CONCEPT_RELATIONSHIP and the ‘Maps to’ relations to identify non-standard concepts that will be contained within the ‘included concepts’ list.
The first point is the one I am just trying to understand: you say ‘find the ingredient and first go up the hierarchy’, does that mean that a RxNorm ingredient will have an ancestor record to ALL ATC concepts which contain that ingredient now? If so, that’s amazing and extraordinarily useful!