Process the commission.

        Parameters
        ----------
        commission : zp.Event
            The commission being paid.

def process_commission(self, commission):
        """Process the commission.

        Parameters
        ----------
        commission : zp.Event
            The commission being paid.
        """
        asset = commission['asset']
        cost = commission['cost']

        self.position_tracker.handle_commission(asset, cost)
        self._cash_flow(-cost)