message: StructureSequence, handle empty coarse hierarchies
diff: @@ -24,6 +24,8 @@ namespace StructureSequence {
  readonly sequence: Sequence
  }
  + const Empty: StructureSequence = { byEntityKey: {}, sequences: [] }
  +
  function merge(...entitySeqs: StructureSequence[]): StructureSequence {
  const sequences: StructureSequence.Entity[] = []
  const byEntityKey: { [key: number]: StructureSequence.Entity } = {}
  @@ -37,7 +39,7 @@ namespace StructureSequence {
  export function fromHierarchy(entities: Entities, atomicHierarchy: AtomicHierarchy, coarseHierarchy: CoarseHierarchy, modResMap?: ReadonlyMap<string, string>): StructureSequence {
  const atomic = fromAtomicHierarchy(entities, atomicHierarchy, modResMap)
  - const coarse = fromCoarseHierarchy(entities, coarseHierarchy)
  + const coarse = coarseHierarchy.isDefined ? fromCoarseHierarchy(entities, coarseHierarchy) : Empty
  return merge(atomic, coarse)
  }
  