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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def pack(self, value=None):
 r"""Pack the value as a binary representation. Considering an example with UBInt8 class, that inherits from GenericType: b'\x01' b'\x05' Args: value: If the value is None, then we will pack the value of the current instance. Otherwise, if value is an instance of the same type as the current instance, then we call the pack of the value object. Otherwise, we will use the current instance pack method on the passed value. Returns: bytes: The binary representation. Raises: :exc:`~.exceptions.BadValueException`: If the value does not fit the binary format. """

        if isinstance(value, type(self)):
            return value.pack()

        if value is None:
            value = self.value
        elif 'value' in dir(value):
            # if it is enum or bitmask gets only the 'int' value
            value = value.value

        try:
            return struct.pack(self._fmt, value)
        except struct.error:
            expected_type = type(self).__name__
            actual_type = type(value).__name__
            msg_args = expected_type, value, actual_type
            msg = 'Expected {}, found value "{}" of type {}'.format(*msg_args)
            raise PackException(msg)