<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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"]