<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def unpack(self, unpacker):
 """ unpacks an exception handler entry in an exception table. Updates the internal structure of this instance """


        (a, b, c, d) = unpacker.unpack_struct(_HHHH)

        self.start_pc = a
        self.end_pc = b
        self.handler_pc = c
        self.catch_type_ref = d