1. Suppose for an application, valid passwords are of length 4 and satisfy the following properties: ˆ Each character must be chosen from the set {a, b, c, d, 0, 1, 2, 3, ?, !}. ˆ There must be at least one letter, one digit, and one special character (neither a digit nor a letter). ˆ The starting character must be a letter, and the last character cannot be a special character. Give a regular grammar that generates the set of all valid passwords for this application