Define a grammar for the empty language, which is a language containing no strings. (There is no valid sentence in the empty language.)

Define a grammar for the letter language. A letter is a lower-case Latin letter between a and z.

Define a grammar for the word language. A word is a sequence of one or more letters. You may refer to the letter nonterminal defined above.

Define a grammar for the parenthesis language, where all sentences in the parenthesis language consist of balanced pairs of left and right parentheses. Example of valid pairs: (), (()), ()(), (((()())()())). Example of invalid pairs: ())(, (, )(, )(((((((((.