getLosslessOB (symbol) {
    if (!this._losslessOrderBooks[symbol]) return null

    return new OrderBook(this._losslessOrderBooks[symbol])
  }
Returns an up-to-date lossless copy of the order book for the specified symbol, or
null if no OB is managed for that symbol. All amounts and prices are in original
string format.

Set `manageOrderBooks: true` in the constructor to use.

@param {string} symbol - symbol for order book
@returns {OrderBook} ob - null if not found