2. Find (as simple as possible) unambiguous context-free grammars which generate the following languages over A - [a, b (a) L {x | x = rev(x) num(b, x) is a multiple of 3), (b) L-[x | if x begins with an a, each sequence of symbols a is followed by the same f: number of symbols b, otherwise each sequence of symbols b is followed by the same number of symbols a). Illustrate each of your grammars by a derivation tree for a nontrivial string