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The union operator : in surface mode is more complex than in body mode. It is almost always used with parentheses. Consider, for example, the cell description 1 2 (3 :-4) and surfaces that are planes with senses as in the previous topic. The region in parentheses then includes all of the points to the right of surface 3 plus all of the points to the left of surface 4. The region to the left of the parentheses is the set of points lying to the right of surface 1 and to the right of surface 2. The cell is then the intersection of those two regions.