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()

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)