code: def set_pkg_source_info(self, doc, text):
          
          self.assert_package_exists()
          if not self.package_source_info_set:
              self.package_source_info_set = True
              doc.package.source_info = text
              return True
          else:
              raise CardinalityError('Package::SourceInfo')
docs: Sets the package's source information, if not already set.
          text - Free form text.
          Raises CardinalityError if already defined.
          Raises OrderError if no package previously defined.