std::vector<string> backedges;
  for (const Edge* backedge : merge->in_edges()) {
    if (backedge->src()->IsNextIteration()) {
      backedges.push_back(absl::StrCat("  ", SummarizeNode(*backedge->src())));
    }
  }
  return errors::InvalidArgument(
      "Multiple NextIteration inputs to merge node ",
      FormatNodeForError(*merge), ": \n", absl::StrJoin(backedges, "\n"),