"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;
  CHECK(merge->IsMerge());
  for (const Edge* e : merge->in_edges()) {
    if (e->src()->IsNextIteration()) {
      if (*result != nullptr) {