<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def to_binary_string(self):
 """Pack the feedback to binary form and return it as string."""

        timestamp = datetime_to_timestamp(self.when)
        token = binascii.unhexlify(self.token)
        return struct.pack(self.FORMAT_PREFIX + '{0}s'.format(len(token)),
                           timestamp, len(token), token)