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The complement operator # is unique to surface mode. It is followed by a cell number, rather than a list of surfaces, and means that the space enclosed within that cell is to be excluded from the cell being described. If a left parenthesis follows the # operator, the space being excluded is a cell described by the Boolean combination of surfaces inside the parentheses. For example, 1 -2 #1 means that the cell being described consists of the space defined by points that have a positive sense with respect to surface 1 and a negative sense with respect to surface 2 but excluding any space in (the previously defined) cell 1. The description 1 -2 #(4 -3) is the same as above except the space being excluded is defined by a Boolean combination of surfaces 4 and 3.