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```import collections

KEY, PREV, NEXT = range(3)

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

if key not in self.map:
end = self.end
curr = end[PREV]
curr[NEXT] = end[PREV] = self.map[key] = [key, curr, end]

if key in self.map:
key, prev, next = self.map.pop(key)
prev[NEXT] = next
next[PREV] = prev

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

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

def pop(self, last=True):
if not self:
raise KeyError('set is empty')
key = next(reversed(self)) if last else next(iter(self))
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)

def __del__(self):
self.clear()                    # remove circular references

if __name__ == '__main__':