Why is the answer 2?

Question

What is the output of the following code?
let a: Set = ["A", "B", "C"]
let b: Set = ["B", "D", "E", "A"]
print(b.subtract(a).count)

Answer 1

b.subtract(a) returns the elements that are contained in b, but not in a, which are the elements D and E.
count returns the number of elements that are returned from the subtract method.