static @Nullable FlatMap create(@NotNull FunctionHelper fn) {
      String varName = fn.tryLightTransform();
      if (varName == null) return null;
      List<OperationRecord> records = extractRecords(fn);
      return records == null ? null : new FlatMap(records, varName, fn);
    }
static @Nullable FlatMap create(@NotNull FunctionHelper fn)