code: func (cert *Certificate) Revoked(tx *sql.Tx, when int64) (bool, error) {
  	if err := cert.Select(tx); err != nil {
  		return true, err
  	}
  
  	var count int
  	row := tx.QueryRow(`SELECT count(*) FROM revocations WHERE ski=? AND revoked_at <= ?`, cert.SKI, when)
  	err := row.Scan(&count)
  	if err != nil {
  		return true, err
  	}
  
  	return count > 0, nil
  }
docs: // Revoked returns true if the certificate was revoked before the
  // timestamp passed in.