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namespace boost { class none_t {/* see below */}; inline constexpr none_t none (/* see below */); } // namespace boost
          Class none_t is meant to
          serve as a tag for selecting appropriate overloads of from optional's interface. It is an empty,
          trivially copyable class with disabled default constructor.
        
          Constant none is used to
          indicate an optional object that does not contain a value in initialization,
          assignment and relational operations of optional.