Optional__kindDiscriminator kind, written on serialization (SohlEntity.toJSON) and read back on revival. Optional as constructor input — it is derived from the concrete class, not supplied by callers.
Faces per die (the M in NdM+K).
Flat modifier (the K in NdM+K).
Number of dice (the N in NdM+K).
Individual die results, if already rolled.
The serializable shape of a SimpleRoll.