Method cancel

include/IECore/Canceller.h:85–97

		}

		void cancel()
		{
			// Store time of first cancellation. We use `compare_exchange_weak()`
			// to avoid unwanted updates on subsequent calls.
			std::chrono::steady_clock::rep epoch( 0 );
			m_cancellationTime.compare_exchange_weak(
				epoch,
				std::chrono::steady_clock::now().time_since_epoch().count()
			);
			// Set cancellation flag _after_ storing time, so that
			// `elapsedTime()` always sees a valid time.
			m_cancelled = true;
		}

		bool cancelled() const
		{