One case of a `[]` DU: the case name, the tag literal when it does not spell the case name, and the arm's own properties as case fields. The fields are the arm's properties *other than* the discriminant - Fable writes the tag itself from the case's compiled name. Verified against Fable 5.13: `Circle(radius = 2.0)` emits `{ kind: "circle", radius: 2 }`, and a `None` in an optional field omits the key rather than writing `undefined`, which is exactly TypeScript's optional-property semantics. Carrying the arm type as a single payload field instead does *not* work - Fable wraps it as `{ kind: "circle", Item: x }`, an object no TypeScript signature would accept.