code: def set_doc_spdx_id(self, doc, doc_spdx_id_line):
          
          if not self.doc_spdx_id_set:
              if doc_spdx_id_line == 'SPDXRef-DOCUMENT':
                  doc.spdx_id = doc_spdx_id_line
                  self.doc_spdx_id_set = True
                  return True
              else:
                  raise SPDXValueError('Document::SPDXID')
          else:
              raise CardinalityError('Document::SPDXID')
docs: Sets the document SPDX Identifier.
          Raises value error if malformed value, CardinalityError
          if already defined.