Subscribe to market depth data (a.k.a. DOM, L2 or order book).

        https://interactivebrokers.github.io/tws-api/market_depth.html

        Args:
            contract: Contract of interest.
            numRows: Number of depth level on each side of the order book
                (5 max).
            isSmartDepth: Consolidate the order book across exchanges.
            mktDepthOptions: Unknown.

        Returns:
            The Ticker that holds the market depth in ``ticker.domBids``
            and ``ticker.domAsks`` and the list of MktDepthData in
            ``ticker.domTicks``.

def reqMktDepth(
            self, contract: Contract, numRows: int = 5,
            isSmartDepth: bool = False, mktDepthOptions=None) -> Ticker:
        """
        Subscribe to market depth data (a.k.a. DOM, L2 or order book).

        https://interactivebrokers.github.io/tws-api/market_depth.html

        Args:
            contract: Contract of interest.
            numRows: Number of depth level on each side of the order book
                (5 max).
            isSmartDepth: Consolidate the order book across exchanges.
            mktDepthOptions: Unknown.

        Returns:
            The Ticker that holds the market depth in ``ticker.domBids``
            and ``ticker.domAsks`` and the list of MktDepthData in
            ``ticker.domTicks``.
        """
        reqId = self.client.getReqId()
        ticker = self.wrapper.startTicker(reqId, contract, 'mktDepth')
        self.client.reqMktDepth(
            reqId, contract, numRows, isSmartDepth, mktDepthOptions)
        return ticker