# Source code for lxml.html._setmixin

    try:
        from collections.abc import MutableSet
    except ImportError:
        from collections.abc import MutableSet

    class SetMixin(MutableSet):

        """
        Mix-in for sets.  You must define __iter__, add, remove
        """

        def __len__(self):
            length = 0
            for item in self:
                length += 1
            return length

        def __contains__(self, item):
            for has_item in self:
                if item == has_item:
                    return True
            return False

        issubset = MutableSet.__le__
        issuperset = MutableSet.__ge__

        union = MutableSet.__or__
        intersection = MutableSet.__and__
        difference = MutableSet.__sub__
        symmetric_difference = MutableSet.__xor__

        def copy(self):
            return set(self)

        def update(self, other):
            self |= other

        def intersection_update(self, other):
            self &= other

        def difference_update(self, other):
            self -= other

        def symmetric_difference_update(self, other):
            self ^= other

        def discard(self, item):
            try:
                self.remove(item)
            except KeyError:
                pass

        @classmethod
        def _from_iterable(cls, it):
            return set(it)