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# Long description

The figure shows two parallel horiontal lines. The upper one is labelled q1 and the lower one is labelled q2. In the middle of the upper line is a large circle with a large plus sign in it. Further along the upper line is a square box with a curved line and an arrow in it. A vertical line leads from the large circle on the upper line to the lower line where it joins a large dot. To the left of this on the lower line a square box with a capital H in it; to the right of the large dot on the lower line is a square box with a curved line and an arrow in it.