Maybe you've taken a 6 out, or in traditional subtraction, maybe we would end up with a negative number or something. And when you subtract a set, if the set you're subtracting from does not have that element, then taking that element out of it doesn't change it. If I start with set A and take a 6, if I take all the 6s out of set A, it doesn't change it. There was no 6 to begin with. I could take all the zebras out of set A. It will not change it. Now, another way to denote the relative complement of set B in A or B subtracted from A is the notation that I'm about to write.