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Start an asynchronous operation to wait for a signal to be delivered.
template< typename SignalToken = default_completion_token_t<executor_type>> DEDUCED async_wait( SignalToken && token = default_completion_token_t< executor_type >());
This function may be used to initiate an asynchronous wait against the signal set. It is an initiating function for an asynchronous operation, and always returns immediately.
          For each call to async_wait(), the completion handler will be called
          exactly once. The completion handler will be called when:
        
asio::error::operation_aborted.
            
                The completion
                token that will be used to produce a completion handler, which
                will be called when the wait completes. Potential completion tokens
                include use_future,
                use_awaitable,
                yield_context,
                or a function object with the correct completion signature. The function
                signature of the completion handler must be:
              
void handler( const asio::error_code& error, // Result of operation. int signal_number // Indicates which signal occurred. );
                Regardless of whether the asynchronous operation completes immediately
                or not, the completion handler will not be invoked from within this
                function. On immediate completion, invocation of the handler will
                be performed in a manner equivalent to using post.
              
void(asio::error_code, int)
          This asynchronous operation supports cancellation for the following cancellation_type
          values:
        
cancellation_type::terminal
            cancellation_type::partial
            cancellation_type::total