anchor.diff:  === ...
   image::art/tree-2649411_1920.jpg[canvas]
   
  -[source,c+]
  -----
  -constexpr auto make_branch(auto left, auto right)
  +[source,c++]
  +----
  +constexpr auto make_branch(auto left, auto right) // -> (fn -> optional<int>)
   {
  -    return [left,right](int key) -> pair<bool,int>
  +
  +    return [left,right](int key) -> optional<int>
       {
  -        if(const auto [success, value] = left(key); success) {
  -	    return {true, value};
  +	const auto result = left(key)
  +        if( result != nullopt )
  +	    return result;
           }
           return right(key);
       };
  
positive.code: `c++11`
  [source,c++]
  ----
  for(auto& item : *self->classes_by_descriptor)
      ...
  }
  ----
  
negative.code: 
  [source,c++]
  ----
  static_assert( nullopt == my_lock(84) );
  static_assert( 1337    == my_lock(42) );
  ----
  
  [state=dimbg]
  