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A customisation point that is used to obtain a sender from a scheduler.
constexpr unspecified schedule = unspecified;
        The name execution::schedule denotes a customisation point object.
        For some subexpression s, let S be a type such
        that decltype((s)) is S. The expression execution::schedule(s)
        is expression-equivalent to:
      
s.schedule(), if that expression is valid and its type models
            sender.
          schedule(s), if that expression is valid and
            its type models sender with overload resolution performed
            in a context that includes the declaration void schedule();
            and that does not include a declaration of execution::schedule.
          S if S satisfies executor.
          execution::schedule(s) is ill-formed.
          
        Header: asio/execution/schedule.hpp
      
        Convenience header: asio/execution.hpp