code: def set_pkg_verif_code(self, doc, code):
          
          self.assert_package_exists()
          if not self.package_verif_set:
              self.package_verif_set = True
              match = self.VERIF_CODE_REGEX.match(code)
              if match:
                  doc.package.verif_code = match.group(self.VERIF_CODE_CODE_GRP)
                  if match.group(self.VERIF_CODE_EXC_FILES_GRP) is not None:
                      doc.package.verif_exc_files = match.group(self.VERIF_CODE_EXC_FILES_GRP).split(',')
                  return True
              else:
                  raise SPDXValueError('Package::VerificationCode')
          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.
          Raises Value error if doesn't match verifcode form