Ocaml programming, please! Don't use List module! Please explain!

combine lst1 lst2

• Type: 'a list -> 'a list -> 'a list
• Description: Returns a list with the elements of lst1 followed by the elements of lst2. The elements within each list must be in the same order. You may not use the @ operator to write this function (that is, in this function or any helper functions).
• Examples:

  combine [] [] = [] combine [] [3; 4] = [3; 4] combine [1; 2; 3; 4] [3; 4; 5] = [1; 2; 3; 4; 3; 4; 5] combine ["a"] ["b"] = ["a"; "b"]