2. The number of ways you can choose r things from a set of n, ignoring the order in which they are chosen, is (?) = n!/(r!(n - r)!). Let a be the first element of the set of n things. We can partition the collection of possible size r subsets into those that contain x and those that don't: there must be (? !) subsets of the first type and (") subsets of the second type. Thus n + r Using this and the fact that (n) = (") = 1, write a function to calculate (). Hint: develop a recursive function. In other words, you use the function within itself]