docs: def convert(amount, source, target='CZK', date=None, percent=0, valid_days_max=None):
      """without target parameter returns equivalent of amount+source in CZK
      with target parameter returns equivalent of amount+source in given currency
      you can calculate with regard to (given) date
      you can add additional margin with percent parameter
      valid_days_max: see rate()
      """
      if source.upper() == 'CZK':
          czk = amount
      else:
          czk = amount * rate(source, date, valid_days_max=valid_days_max)
      result = convert_to(target, czk, date, valid_days_max=valid_days_max)
      return modified(result, percent)