Suppose a broker must choose to invest in four investments 1, 2, 3, and 4. Let x1, x2, x3, and x4 denote the binary variables if she chooses the particular investment or not. Suppose she has the following constraints: If she invests in 3 or 4 or both then she must invest in exactly one of 1 or 2. Otherwise, if she invests in neither (of 3 or 4) then there are no constraints on investing in 1 or 2.