Show that if $M=\left(S, I, f, s_{0}, F\right)$ is a deterministic finite-state automaton and $f(s, x)=s$ for the state $s \in S$ and the input string $x \in I^{*},$ then $f\left(s, x^{n}\right)=s$ for every nonnegative integer $n .\left(\text { Here } x^{n} \text { is the concatenation of } n \text { copies }\right.$ of the string $x,$ defined recursively in Exercise 37 in Section $5.3 . )$