<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def balance_of(self, address, block_identifier='latest'):
 """ Return the balance of `address`. """

        return self.proxy.contract.functions.balanceOf(
            to_checksum_address(address),
        ).call(block_identifier=block_identifier)