Natural_deduction : Main.conclusion_of

let conclusion_of_prf (prf : t_prf) : t_fml =
        match prf with
        |Atomic_prf fml -> fml
        |Nullary_prf (_, fml) -> fml
        |Unary_prf (_,_,fml) -> fml
        |Binary_prf (_,_,_,fml) -> fml
        |Trinary_prf (_,_,_,_,fml) -> fml