(5 points) Below is a recursive deinition for a set S of strings over the alphabet a, b). The set S does not include all strings over the alphabet {a, b}. . Basis: X ES and a E S . Recursive rules: If r is a string and r E S, thern - xbES - xba E S . The only strings that are in S are the ones that can be constructed by starting with one of the strings in the base case and repeatedly applying the recursive rules above. List all the strings of length 3 in S