Variant constructor record returned by defineMsgs. For
each tag the spec defined as null, the entry is a zero-arg
function returning { kind }. For each tag with a payload marker,
the entry takes the payload object and returns { kind, ...payload }.
Phantom field carrying the closed Msg union for the spec.
Read with MsgOf. Always undefined at runtime.
Remarks
The record also carries a phantom _union field whose type is the
closed Msg union for the spec. MsgOf reads that field; it
exists only at the type level and is undefined at runtime.
Variant constructor record returned by defineMsgs. For each tag the spec defined as
null, the entry is a zero-arg function returning{ kind }. For each tag with a payload marker, the entry takes the payload object and returns{ kind, ...payload }.