class Deque:
    def add_first(self, value):
        ...

    def add_last(self, value):
        ...

    def delete_first(self):
        ...

    def delete_last(self):
        ...

    def first(self):
        ...

    def last(self):
        ...

    def is_empty(self):
        ...

    def __len__(self):
        ...

    def __str__(self):
        ...

[]