We suppose that a finite alphabet is given, and let word x $=$ x$1$x$2...$xn and y $=$ y$1$y$2...$ ym be strings over $.$ We use $($y$)$ to denote the set of strings over obtained by repeating y$.$ For instance, if y $=$ AB$,$ then $($AB$)=\{,$ AB$,$ ABAB, ABABAB, $...\}$ and ABA $/($AB$)$