Q:

Why isn't there a formSubtracting() method in Swift for Sets

In Swift there is a Form... equivalent for the Sets methods intersection(), symmetricDifference() and union(), i.e. formIntersection(), formSymmetricDifference() and formUnion(). 
But for the method subtracting() there is no method called formSubtracting. Does anyone know why this is so, because it seams I now have to use something like mySet = mySet.subtracting(anotherSet)

A:

subtract(_:) is what you are looking for:

Removes the elements of the given set from this set.

Example:
var mySet: Set = [1, 2, 3, 4, 5]
let anotherSet : Set = [2, 4, 6, 8]
mySet.subtract(anotherSet)
print(mySet) // [3, 1, 5]

There is also a variant which takes another sequence (of the same element type) as the argument, e.g. an array:
var mySet: Set = [1, 2, 3, 4, 5]
let anotherSequence = [2, 4, 6, 8]
mySet.subtract(anotherSequence)
print(mySet) // [3, 1, 5]

