text: // Copyright 2021 Francois Chabot
  
  // Licensed under the Apache License, Version 2.0 (the "License");
  // you may not use this file except in compliance with the License.
  // You may obtain a copy of the License at
  
  //     http://www.apache.org/licenses/LICENSE-2.0
  
  // Unless required by applicable law or agreed to in writing, software
  // distributed under the License is distributed on an "AS IS" BASIS,
  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  // See the License for the specific language governing permissions and
  // limitations under the License.
  
  #ifndef ABU_FEED_TAGS_H_INCLUDED
  #define ABU_FEED_TAGS_H_INCLUDED
  
  #include <concepts>
  
  namespace abu::feed {
  
  // ***** Universal feed sentinels *****
  
  // This sentinel denotes that the feed has run out of data
  struct empty_feed_t {};
  constexpr empty_feed_t empty;
  
  // This sentinel denotes that the feed has reached its end.
  struct end_of_feed_t {};
  constexpr end_of_feed_t end_of_feed;
  
  }  // namespace abu::feed
  
  #endif