Basically it is just a flag: if True, the values in c.levels are seen as increasing.

```
data = [1,3,4,5,1,3]
c = Categorical(data, levels=[1,5,3,4], orderd=True) # ordered, 1 < 5 < 3 < 4
c = Categorical(data, levels=[1,5,3,4]) # not ordered

The interesting thing is that in the first case, c.sort() should return something like `[1,1,5,3,4]`:

```R
> fert = c(10,20,20,50,10,20,10,50,20)
> fert = factor(fert,levels=c(50,20,10),ordered=TRUE)
> fert
[1] 10 20 20 50 10 20 10 50 20
Levels: 50 < 20 < 10
> sort(fert)
[1] 50 50 20 20 20 20 10 10 10
Levels: 50 < 20 < 10
```