'Returns (price, price_level) pair where price is predecessor to given price
:type price: Price
:rtype: (Price, PriceLevel)'

def prev_item(self, price):

    assert isinstance(price, Price), type(price)
    index = (self._price_list.index(price) - 1)
    if (index < 0):
        raise IndexError
    prev_price = self._price_list[index]
    return (prev_price, self._price_level_dictionary[prev_price])
