Problem #$4.$ Give a recursive de$$nition for the set of all strings of a$$s and b$$s where all the strings contain exactly two a$\text{'}$s and they must be consecutive. $($Assume$,$ S is set of all strings of a$$s and b$$s where all the strings contain exactly two consecutive a$\text{'}$s$.$ Then S $=\{$aa$,$ aab, baa, aabb, baab, baab, bbaa, aabbb, baabb, $...\}).$