'Update the set, adding any elements from other which are not
already in the set.
@param other: the collection of items with which to update the set
@type other: Set object'

def union_update(self, other):

    if (not isinstance(other, Set)):
        raise ValueError('other    must    be    a    Set    instance')
    if (self is other):
        return
    for item in other.items:
        self.add(item)
