'Add an item to the set.'

def add(self, item):

    oid = id(item)
    if (oid in self._items):
        self._order.remove(oid)
        self._order.append(oid)
        return
    ref = weakref.ref(item, (lambda x: self._remove_by_id(oid)))
    self._items[oid] = ref
    self._order.append(oid)
