code: def marginBuy(self, currencyPair, rate, amount, lendingRate=None):
          
          return self._private('marginBuy', currencyPair=currencyPair, rate=rate,
                               amount=amount, lendingRate=lendingRate)
docs: Places a margin buy order in a given market. Required POST
          parameters are "currencyPair", "rate", and "amount". You may optionally
           specify a maximum lending rate using the "lendingRate" parameter.
           If successful, the method will return the order number and any trades
           immediately resulting from your order.