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# Source code for pydna.orderedset

    import collections

    class OrderedSet(collections.MutableSet):

        def __init__(self, iterable=None):
            self.end = end = []
            end += [None, end, end]         # sentinel node for doubly linked list
            self.map = {}                   # key --> [key, prev, next]
            if iterable is not None:
                self |= iterable

        def __len__(self):
            return len(self.map)

        def __contains__(self, key):
            return key in self.map

        def add(self, key):
            if key not in self.map:
                end = self.end
                curr = end[1]
                curr[2] = end[1] = self.map[key] = [key, curr, end]

        def discard(self, key):
            if key in self.map:
                key, prev, next = self.map.pop(key)
                prev[2] = next
                next[1] = prev

        def __iter__(self):
            end = self.end
            curr = end[2]
            while curr is not end:
                yield curr[0]
                curr = curr[2]

        def __reversed__(self):
            end = self.end
            curr = end[1]
            while curr is not end:
                yield curr[0]
                curr = curr[1]

        def pop(self, last=True):
            if not self:
                raise KeyError('set is empty')
            key = self.end[1][0] if last else self.end[2][0]
            self.discard(key)
            return key

        def __repr__(self):
            if not self:
                return '%s()' % (self.__class__.__name__,)
            return '%s(%r)' % (self.__class__.__name__, list(self))

        def __eq__(self, other):
            if isinstance(other, OrderedSet):
                return len(self) == len(other) and list(self) == list(other)
            return set(self) == set(other)

    if __name__ == '__main__':
        s = OrderedSet('abracadaba')
        t = OrderedSet('simsalabim')
        print(s | t)
        print(s & t)
        print(s - t)

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