backedges.push_back(absl::StrCat("  ", SummarizeNode(*backedge->src())));
    }
  }
  return errors::InvalidArgument(
      "Multiple NextIteration inputs to merge node ",
      FormatNodeForError(*merge), ": \n", absl::StrJoin(backedges, "\n"),
      "\nMerge nodes can have at most one incoming NextIteration edge.");
}

Status FindUniqueBackedge(Node* merge, const Edge** result) {
  *result = nullptr;