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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def to_notional(instruments, prices, multipliers, desired_ccy=None, instr_fx=None, fx_rates=None):
 """ Convert number of contracts of tradeable instruments to notional value of tradeable instruments in a desired currency. Parameters instruments: pandas.Series Series of instrument holdings. Index is instrument name and values are number of contracts. prices: pandas.Series Series of instrument prices. Index is instrument name and values are instrument prices. prices.index should be a superset of instruments.index otherwise NaN returned for instruments without prices multipliers: pandas.Series Series of instrument multipliers. Index is instrument name and values are the multiplier associated with the contract. multipliers.index should be a superset of instruments.index desired_ccy: str Three letter string representing desired currency to convert notional values to, e.g. 'USD'. If None is given currency conversion is ignored. instr_fx: pandas.Series Series of instrument fx denominations. Index is instrument name and values are three letter strings representing the currency the instrument is denominated in. instr_fx.index should match prices.index fx_rates: pandas.Series Series of fx rates used for conversion to desired_ccy. Index is strings representing the FX pair, e.g. 'AUDUSD' or 'USDCAD'. Values are the corresponding exchange rates. Returns ------- pandas.Series of notional amounts of instruments with Index of instruments names Example ------- """

    notionals = _instr_conv(instruments, prices, multipliers, True,
                            desired_ccy, instr_fx, fx_rates)
    return notionals