public ValueTask <ImmutableArray <TextSpan> > GetSpansAsync(
     Document document,
     CancellationToken cancellationToken
     ) =>
 _session?.GetSpansAsync(document, cancellationToken)
 ?? new(ImmutableArray <TextSpan> .Empty);
 public Task <ImmutableArray <TextSpan> > GetSpansAsync(Document document, CancellationToken cancellationToken)
 => _session?.GetSpansAsync(document, cancellationToken) ?? SpecializedTasks.EmptyImmutableArray <TextSpan>();