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          An asynchronous operation has an associated executor satisfying the Executor requirements.
          If not otherwise specified by the asynchronous operation, this associated
          executor is an object of type system_executor.
        
All asynchronous operations in this library have an associated executor object that is determined as follows:
          — If the initiating function is a member function, the associated executor
          is that returned by the get_executor member function on the
          same object.
        
          — If the initiating function is not a member function, the associated executor
          is that returned by the get_executor member function of the
          first argument to the initiating function.
        
          Let Executor1 be the type of the associated executor. Let
          ex1 be a value of type Executor1, representing
          the associated executor object obtained as described above.