Request CDA namespace prefix be added to support Australian CDA transformations

[robeastwood-agency]

The Australian Digital Health Agency is commencing an exploration of transforming FHIR content (ie document bundles) into CDA content, specifically making use of Australian CDA extensions [1]. A working CDA logical model, adapted from the HL7 International CDA Logical Model Specification, is being used to represent the AU CDA extensions as the basis for successful transforms from FHIR Bundles to CDA instances. Transforms are being performed by the Matchbox server as well as the FHIR Validator.

Generated CDA instances however are populated with the namespace prefix ns0 instead of the defined prefix ext; as such:

<ClinicalDocument xmlns="urn:hl7-org:v3"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:ns0="http://ns.electronichealth.net.au/Ci/Cda/Extensions/3.0"">
  ...
  <ns0:completionCode code="F" codeSystem="1.2.36.1.2001.1001.101.104.20104" displayName="Final"/>
  ...

@oliveregger has indicated that the namespace prefix is controlled by this repo, specifically these lines.

Therefore, this request is to add the following content to support the auto-generation of the Australian namespace extension prefix ext:

if (ns.equals("http://ns.electronichealth.net.au/Ci/Cda/Extensions/3.0"))
      return "ext:";

In support of this, I will shortly submit a PR for the above changes.

Thank you

Notes
[1] - https://developer.digitalhealth.gov.au/resources/australian-digital-health-agency-cda-schema-extension-3-0-cda-schema-v20201203