@Override
  public @NotNull DocumentContent create(@Nullable Project project, @NotNull Document document, @Nullable DocumentContent referent) {
    if (referent == null) return new DocumentContentImpl(document);
    return new ContentReferentDocumentContent(project, document, referent);
  }
@Override
  public @NotNull DocumentContent create(@Nullable Project project, @NotNull Document document, @Nullable DocumentContent referent)