datatype expression =  Constant of int |
        Variable of string |
        Operator of string * expression |
        Pair of expression list |
        List of expression list

datatype pattern = ConstantP of int
    | VariableP of string
    | OperatorP of string * pattern
    | PairP of pattern list
    | ListP of pattern list
    | UnorderedListP of pattern list
    | Wildcard


fun pretvori(pair: pattern) =
    case pair of
        VariableP a => Variable a
        | ConstantP a => Constant a
        | PairP [a,b] => Pair [pretvori(a), pretvori(b)]
        | OperatorP(a, b) => Operator(a, (pretvori(b)))
        | ListP a => case a of
            h::nil => List [pretvori(h)]
            |h::t => List ([pretvori(h)] @ [] @ [pretvori(ListP t)])

pretvori(ListP([ConstantP 1, ConstantP 2, ListP([ConstantP 3, ConstantP 7, ListP([ConstantP 77, ConstantP 33])])]));

(*OUTPUT*)
val it =
  List
    [Constant 1,
     List
       [Constant 2,
        List
          [List
             [Constant 3,
              List [Constant 7,List [List [Constant 77,List [Constant 33]]]]]]]]
  : expression

(*WHAT THE OUTPUT SHOULD BE *)
pretvori(List([Constant 1, ConstantP , List([Constant 3, Constant 7, List([Constant 77, Constant 33])])]));