code: def set_doc_version(self, doc, value):
          
          if not self.doc_version_set:
              self.doc_version_set = True
              m = self.VERS_STR_REGEX.match(value)
              if m is None:
                  raise SPDXValueError('Document::Version')
              else:
                  doc.version = version.Version(major=int(m.group(1)),
                                                minor=int(m.group(2)))
                  return True
          else:
              raise CardinalityError('Document::Version')
docs: Set the document version.
          Raise SPDXValueError if malformed value, CardinalityError
          if already defined