The subtracting method returns a unique set of elements that can be found in the set the method was called on, but not in the set that was passed as a parameter. Unlike the other set manipulation methods we've mentioned, this will not necessarily return the same value if you swap the set that the method is called on with the set provided as a parameter:
let squareNotOdd = squareNumbers.subtracting(oddNumbers) // 4
The following diagram depicts the set that's created by Subtracting Set B from Set A:
Figure 2.5– Subtracting a set
