docs: 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
      -------
      >>> import pandas as pd
      >>> import mapping.util as util
      >>> current_contracts = pd.Series([-1, 1], index=['CLX16', 'CLZ16'])
      >>> prices = pd.Series([50.32, 50.41], index=['CLX16', 'CLZ16'])
      >>> multipliers = pd.Series([100, 100], index=['CLX16', 'CLZ16'])
      >>> ntln = util.to_notional(current_contracts, prices, multipliers)
      """
      notionals = _instr_conv(instruments, prices, multipliers, True,
                              desired_ccy, instr_fx, fx_rates)
      return notionals