code: def has_vary_header(response, header_query):
      
      if not response.has_header('Vary'):
          return False
      vary_headers = cc_delim_re.split(response['Vary'])
      existing_headers = set([header.lower() for header in vary_headers])
      return header_query.lower() in existing_headers
docs: Checks to see if the response has a given header name in its Vary header.