Please code in OCaml language and without using any list library methods. This method should only be coded using standard library methods, thank you.

flatten lst - Type: 'a list list ' ' list - Description: Returns a combined list of all of the lists in lst. Make sure the original ordering of the lists in lst is preserved. - Examples: flatten [[1;2;3;4];[5;6]]=[1;2;3;4;5;6] flatten [[6];[5];[4];[3];[2];[1]]=[6;5;4;3;2;1] flatten [[1;2];[3;4];[]]=[1;2;3;4] flatten [[1];[2];[3]=[1;2;3]