add Message.preview()

            for d in dlist:

                f.write(self.dict2msg(d) + "\n")

    def preview(self, dlist: list[dict]) -> str:

        """Preview messages."""

        s = ""

        for d in dlist:

            s += self.dict2msg(d) + "\n"

        print(s)

    @staticmethod

    def dict2msg(d: dict):

        """Convert `dict` to JSON format string."""