code: func Matches(c context.Context, host, zone, proj string) bool {
  	p := getPayload(c)
  	if p == nil {
  		return false
  	}
  	logging.Debugf(c, "expecting VM token from %q in %q in %q", host, zone, proj)
  	return p.Instance == host && p.Zone == zone && p.Project == proj
  }
docs: // Matches returns whether the current context contains a GCE VM metadata
  // token matching the given identity.