Define a function single 'a list list -> 'a list which takes as input, a list of lists, and returns a list of an original elements in the same order in which they appear in the argument. Examples: single [] = [] single [[]] = [] single [[1,2], [1, 3, 4], [], [5], [6,7]] single [["a"],["b", "c"]] ["a", "b", "c"]] = = [1,2,1,3,4,5,6,7]