Comment:

The chosen approach to extensions will lead to unnecessary significant increased size of FHIR artifacts. It is expected that a fair number of extension will be required for a particular use case that is not generic. Each single data element that is to be extended will have to have the full URI specified. In addition to increasing the size, it also makes the resources significantly more difficult to read and understand. I would recommend strongly considering the W3C Resource Description Framework (RDF) standard for creation of extensions in XML and JSON LD for representation in JSON. Both of these methods are more lean and readable than the current proposal.