code: func lookupAuthStyle(tokenURL string) (style AuthStyle, ok bool) {
  	authStyleCache.Lock()
  	defer authStyleCache.Unlock()
  	style, ok = authStyleCache.m[tokenURL]
  	return
  }
docs: // lookupAuthStyle reports which auth style we last used with tokenURL
  // when calling RetrieveToken and whether we have ever done so.