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Cancel all operations associated with the signal set.
asio::error_code cancel( asio::error_code & ec);
            This function forces the completion of any pending asynchronous wait
            operations against the signal set. The handler for each cancelled operation
            will be invoked with the asio::error::operation_aborted
            error code.
          
Cancellation does not alter the set of registered signals.
Set to indicate what error occurred, if any.
            If a registered signal occurred before cancel() is called, then the handlers for asynchronous
            wait operations will:
          
These handlers can no longer be cancelled, and therefore are passed an error code that indicates the successful completion of the wait operation.