code: func (ssa *SQLStorageAuthority) checkFQDNSetExists(selector oneSelectorFunc, names []string) (bool, error) {
  	var count int64
  	err := selector(
  		&count,
  		`SELECT COUNT(1) FROM fqdnSets
  		WHERE setHash = ?
  		LIMIT 1`,
  		hashNames(names),
  	)
  	return count > 0, err
  }
docs: // checkFQDNSetExists uses the given oneSelectorFunc to check whether an fqdnSet
  // for the given names exists.