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

def succ_item(self, price):

    assert isinstance(price, Price), type(price)
    index = (self._price_list.index(price) + 1)
    if (index >= len(self._price_list)):
        raise IndexError
    succ_price = self._price_list[index]
    return (succ_price, self._price_level_dictionary[succ_price])
