auto attr = n.attrs().Find(attr_name);
  if (!attr) {
    return std::nullopt;
  } else {
    return attr->s();
  }
}

// Adds a value to the node's list attribute.
template <typename T>
Status AppendToListAttr(Node* n, const string& attr_name, const string& value) {
  std::vector<T> attr_value;
  Status s = GetNodeAttr(n->attrs(), attr_name, &attr_value);
  if (!s.ok() && s.code() != error::NOT_FOUND) {
    return s;
  }