Entity identity (typically a GlobalId) in the base revision.
Entity identity in the head revision — "here".
Human-readable provenance of the claim. Entries minted by
identityMapFromContentMatches use
`${CONTENT_MATCH_REASON_PREFIX}${kind}` so a reviewer (and a filter)
can tell an engine-derived claim from a hand-authored one, and can tell
which evidence produced it.
One identity claim: "the entity known as base in the base revision is the entity known as here in this one".
Structurally identical to
IdentityMapEntryin@ifc-lite/ifcx(packages/ifcx/src/provenance.ts), which is what a published layer carries inidentity_map.