Complete below functions in Ocaml:

let rec filter (f : 'a->bool) (l : 'a list) : 'a list = (* TODO, replace l *) l

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let rec fold_left (f: 'y ->'x->'y) (y:'y) (l:'x list) : 'y = (* TODO, replace y *) y

let rec fold_right (f : 'x->'y->'y) (y:'y) (l:'x list) : 'y = (* TODO, replace y *) y

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(* Concatenate a list of lists. *) let flatten (l : 'a list list) : 'a list = (* TODO, replace [] *) []

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(* Insert elt into sorted list l in sorted order *) let rec insert (cmp : 'a->'a->bool) (elt :'a) (l:'a list) : 'a list = (* TODO, replace [] *) []