<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def returnOrderBook(self, currencyPair='all', depth='50'):
 """Returns the order book for a given market, as well as a sequence number for use with the Push API and an indicator specifying whether the market is frozen. You may set currencyPair to "all" to get the order books of all markets."""

        return self._public('returnOrderBook', currencyPair=currencyPair,
                            depth=depth)