'Update the set, removing any elements from other which are in
the set.
@param other: the collection of items with which to update the set
@type other: Set object'

def difference_update(self, other):

    if (not isinstance(other, Set)):
        raise ValueError('other    must    be    a    Set    instance')
    if (self is other):
        self.items = []
    else:
        for item in other.items:
            self.discard(item)
