The iter part I get, but I don't see how the caller can affect a or b. Whatever value supplied for a or b will be replaced by values in dependent of a and b dur

The iter part I get, but I don't see how the caller can affect a or b. Whatever value supplied for a or b will be replaced by values in dependent of a and b during the loop. a is a function of S and x (which initially is a function of S) and b is a function of x and the just revised value for a.