'Returns the slipped price per share for an order.
:param order: The order being filled.
:type order: :class:`pyalgotrade.broker.Order`.
:param price: The price for each share before slippage.
:type price: float.
:param quantity: The amount of shares that will get filled at this time for this order.
:type quantity: float.
:param bar: The current bar.
:type bar: :class:`pyalgotrade.bar.Bar`.
:param volumeUsed: The volume size that was taken so far from the current bar.
:type volumeUsed: float.
:rtype: float.'

@abc.abstractmethod
def calculatePrice(self, order, price, quantity, bar, volumeUsed):

    raise NotImplementedError()
