procedure TForm1.Button1Click(Sender: TObject);
type
  MySetType = Set of Byte;
Var
  A,B,C : MySetType;
begin
  A := [1,2,3];
  B := [2,3,4];
  C := A B;
  ShowMessage(???);
  end;

procedure TForm1.Button1Click(Sender: TObject);
type
  TMySetType = Set of Byte;
var
  A,B,C : TMySetType;

  function MySetTypeToString(ASet: TMySetType): string;
  var
    L: Byte;
  begin
    if ASet = [] then Result := '[]'
    else begin
      Result := '['
      for L := Low(L) to High(L) do
        if L in ASet then
          Result := Result   Format('%d,', [L]);
      Result[Length(Result)] := ']';
    end;
  end;    begin
  A := [1,2,3];
  B := [2,3,4];
  C := A B;      ShowMessage(MySetTypeToString(C));      end;