code: def set_pkg_verif_code(self, doc, code):
          
          self.assert_package_exists()
          if not self.package_verif_set:
              self.package_verif_set = True
              doc.package.verif_code = code
          else:
              raise CardinalityError('Package::VerificationCode')
docs: Sets the package verification code, if not already set.
          code - A string.
          Raises CardinalityError if already defined.
          Raises OrderError if no package previously defined.