Write a grammar for the language consisting of strings built only of the letters a and b. The strings may have any number of these letters, but the letter combinations bab must be in each string somewhere, and each string must start with a bb. For example, the strings bbababbbaa, bbbaaaababaa, and bbbab are in the language, while a, aabb, baaa, and bbba are not. The initial bb and the in-the-middle bab do not overlap. Neither bab nor bbab are in the language.