So the difference operator in the relation algebra, it basically finds the set difference. If you've learned set theory before, the set difference basically takes a set, r, and then subtract, remove all the elements in set s that exist in r. And that’s basically the result. So think of it as you have the basic relation is r and you have s as a relation. You look for every tuple that exists in s and you cross it off in r. And this is how you retrieve the final answer for the difference operative relation algebra. So an example of that is that you want to list the students for computer science majors who are not double-majoring in electrical engineering. So in this case, you will retrieve the table that has the computer science students. And then you do a said difference between this table and the table that represent the electrical engineering students. So this way, you're going to remove from the computer science table all the students that are also double-majoring in electrical engineering. So this is basically a simple, so in this way, based on the schema we discussed before, so we do a cse_majors from electrical engineering majors, that way. And the output, again, as a rule, relational algebra, the output of any relational algebra, the input is of relation, or two relations, and the output is another relation. So if we do this on the example we've seen earlier, these are the set of students that are, That result of the r- s, which is the difference operator. These are the students that are computer science major but are not double-majoring in electrical engineering. Thank you.