code: def _parse_request_headers(request: web.Request):
          
          headers = request.headers.get(hdrs.ACCESS_CONTROL_REQUEST_HEADERS)
          if headers is None:
              return frozenset()
  
          # FIXME: validate each header string, if parsing fails, raise
          # HTTPForbidden.
          # FIXME: check, that headers split and stripped correctly (according
          # to ABNF).
          headers = (h.strip(" \t").upper() for h in headers.split(","))
          # pylint: disable=bad-builtin
          return frozenset(filter(None, headers))
docs: Parse Access-Control-Request-Headers header or the preflight request
  
          Returns set of headers in upper case.