code: def set_pkg_cr_text(self, doc, text):
          
          self.assert_package_exists()
          if not self.package_cr_text_set:
              self.package_cr_text_set = True
              if validations.validate_pkg_cr_text(text):
                  if isinstance(text, string_types):
                      doc.package.cr_text = str_from_text(text)
                  else:
                      doc.package.cr_text = text  # None or NoAssert
              else:
                  raise SPDXValueError('Package::CopyrightText')
          else:
              raise CardinalityError('Package::CopyrightText')
docs: Sets the package's copyright text.
          Raises OrderError if no package previously defined.
          Raises CardinalityError if already set.
          Raises value error if text is not one of [None, NOASSERT, TEXT].