Natural_deduction : ITM_main.string_of

let string_of_item (item : t_itm) : string =
        match item with
        |Prf prf -> PRF_main.string_of_prf prf
        |Def_fml (left, right) -> String.concat "" [FML_main.string_of_fml left;" := ";FML_main.string_of_fml right]
        |Def_prf (left, right) -> String.concat "" [PRF_main.string_of_prf left;" :=\n\n";PRF_main.string_of_prf right]
        |Comment s -> s