docs: 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"]