(* rimuovi: 'a -> 'a list -> 'a list *)
let rec rimuovi x lst =
    match lst with
        [] -> []
        | el::rest -> if el=x then rest
                      else el::(rimuovi x rest);;

(* path_coprente: 'a tree -> 'a list -> 'a list *)
let rec path_coprente t lst =
    match t with
        Empty -> failwith "err"
        | Tr(x,Empty,Empty) -> if lst=[] || lst=[x] then [x]
                               else failwith "err"
        | Tr(x,l,r) -> if List.mem x lst then x::(try path_coprente l (rimuovi x lst)
                                                  with _ -> path_coprente r (rimuovi x lst))