18. [5 points] Give a BNF grammar for each of the following language: the set of all strings consisting of odd number of concatenated copies of the string "abc 19. [5 points] write an ML function red3 of type , a , c that converts a * 'b , c-> , a with three elements into one with two by eliminating the second element. * * 20. [5 points] Write an ML function thirds of type string -> char that returns the third character of a string. 21. [10 points] Write an ML recursive function pow of type real int -> real that raises a real number to an integer power. 22. [10 points] Write an ML recursive functionlistProduct of type int list -> int to calculate the product of all elements in a list. 23. [10 points] Write an ML function min of type int list ->int that returns the smallest element of a list of integers. Hint: write a helper function element seen so far. Then you can complete the exercise by defining minhelper that takes as a second parameter the smallest fun min x minhelper (tl x, hd x)