We suppose that a finite alphabet $\backslash$Sigma is given, and let word x $=$ x$1$x$2...$ xn and y $=$

y$1$y$2...$ ym be strings over $\backslash$Sigma $.$ We use $($y$)$

$$

to denote the set of strings over $\backslash$Sigma obtained

by repeating y$.$ For instance, if y $=$ AB$,$ then $($AB$)$

$=\{,$ AB$,$ ABAB, ABABAB, $...\}$

and ABA $/$ in $($AB$)$

$.$