Define an unambiguous context free grammar that generates the language of strings containing a series of a's followed by a series of b's followed by a series of c's concluding with a series of d's. The number of a's must be exactly the sames as the number of d's and the number of b's must be exactly the same as the number of c's. The strings must contain at least of instance one each of the four letters. Examples of valid stings are:

Question

aaabbccddd abbbcccd