returns for identical codes (reflexivity broken) on LOINC, UCUM, BCP-47, ISO-3166

[jmandel]

Bug

When the same code is provided as both codeA and codeB, the server should always return equivalent (reflexivity). For four code systems, it incorrectly returns not-subsumed. SNOMED CT works correctly.

Code System	codeA=codeB	Expected	Actual
LOINC	1963-8	equivalent	not-subsumed
UCUM	kg	equivalent	not-subsumed
BCP-47	en	equivalent	not-subsumed
ISO-3166	US	equivalent	not-subsumed
SNOMED CT	22298006	equivalent	equivalent (correct)

Root cause is likely that the subsumption logic doesn't check code equality before attempting hierarchy traversal, and for systems without hierarchy support it falls through to not-subsumed.

Repro

# LOINC — returns not-subsumed (WRONG)
curl -s 'https://tx-dev.fhir.org/r4/CodeSystem/$subsumes?system=http://loinc.org&codeA=1963-8&codeB=1963-8' \
  -H 'Accept: application/fhir+json'

# UCUM — returns not-subsumed (WRONG)
curl -s 'https://tx-dev.fhir.org/r4/CodeSystem/$subsumes?system=http://unitsofmeasure.org&codeA=kg&codeB=kg' \
  -H 'Accept: application/fhir+json'

# SNOMED — returns equivalent (CORRECT, for comparison)
curl -s 'https://tx-dev.fhir.org/r4/CodeSystem/$subsumes?system=http://snomed.info/sct&codeA=22298006&codeB=22298006' \
  -H 'Accept: application/fhir+json'

Spec reference

Subsumption is reflexive by definition — every concept subsumes itself. The outcome equivalent means "A subsumes B and B subsumes A", which must always be true when A=B. The fix is simple: check codeA === codeB before hierarchy lookup.