code: def _process_allow_headers(self, req, resp, requested_headers):
          
          if not requested_headers:
              return True
          elif self._cors_config['allow_all_headers']:
              self._set_allowed_headers(resp, requested_headers)
              return True
  
          approved_headers = []
          for header in requested_headers:
              if header.lower() in self._cors_config['allow_headers_list']:
                  approved_headers.append(header)
              elif self._cors_config.get('allow_headers_regex'):
                  if self._cors_config['allow_headers_regex'].match(header):
                      approved_headers.append(header)
  
          if len(approved_headers) == len(requested_headers):
              self._set_allowed_headers(resp, approved_headers)
              return True
  
          return False
docs: Adds the Access-Control-Allow-Headers header to the response,
          using the cors settings to determine which headers are allowed.
  
          Returns:
              True if all the headers the client requested are allowed.
              False if some or none of the headers the client requested are allowed.