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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def reqPnL(self, account: str, modelCode: str = '') -> PnL: """ Start a subscription for profit and loss events. Returns a :class:`.PnL` object that is kept live updated. The result can also be queried from :meth:`.pnl`. https://interactivebrokers.github.io/tws-api/pnl.html Args: account: Subscribe to this account. modelCode: If specified, filter for this account model. """

        key = (account, modelCode)
        assert key not in self.wrapper.pnlKey2ReqId
        reqId = self.client.getReqId()
        self.wrapper.pnlKey2ReqId[key] = reqId
        pnl = PnL(account, modelCode)
        self.wrapper.pnls[reqId] = pnl
        self.client.reqPnL(reqId, account, modelCode)
        return pnl