Move data between internal fields, validate them and make sure, that
        everything is as it should be.

def _postprocess(self):
        """
        Move data between internal fields, validate them and make sure, that
        everything is as it should be.
        """
        # validate series ISBN
        self._POST["P0601010__a"] = self._validate_isbn(
            self._POST["P0601010__a"],
            accept_blank=True
        )
        if self._POST["P0601010__a"] != "":
            self._POST["P0601010__b"] = "soubor : " + self._POST["P0601010__a"]

        # validate ISBN of the book
        self._POST["P0501010__a"] = self._validate_isbn(
            self._POST["P0501010__a"],
            accept_blank=False
        )
        self._POST["P1601ISB__a"] = self._POST["P0501010__a"]