Exercises should be completed using the OCaml programming language with code presented.

Unless explicitly mentioned, built-in functions or special forms should not be used. If you wish, however, you may implement helper functions

3. concatenate: Given a list of lists, concatenates all the elements of the argument lists into a single list. E.g., Your implementation should be tail-recursive. Additionally, when building your result list, you should only use cons (::), and your implementation should have linear runtime w.r.t. the total number of elements. Hint: it might be helpful to use List.rev at some point in your implementation