<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def satoshi_to_currency(num, currency):
 """Converts a given number of satoshi to another currency as a formatted string rounded down to the proper number of decimal places. :param num: The number of satoshi. :type num: ``int`` :param currency: One of the :ref:`supported currencies`. :type currency: ``str`` :rtype: ``str`` """

    return '{:f}'.format(
        Decimal(
            num / Decimal(EXCHANGE_RATES[currency]())
        ).quantize(
            Decimal('0.' + '0' * CURRENCY_PRECISION[currency]),
            rounding=ROUND_DOWN
        ).normalize()
    )