2. Find (as simple as possible) unambiguous context-free grammars which generate the following languages over A = {a, b}: (a) L (b) L = = {x | x=rev(x) A num(b, x) is a multiple of 3}, {x | if x begins with an a, each sequence of symbols a is followed by the same 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.