bagof/3 same problem as setof/3

```
?- L=[_,_,_,_], bagof(t,permutation(L,L),"tttttt").
   false, unexpected.
   L = [_A,_A,_A,_A].
```

And in any case, the simplest way to implement `setof/3` is:

```
setof(Templ, G_0, Ts) :-
   can_be(list, Ts),
   bagof(Templ, G_0, TsB),
   sort(TsB, Ts).
```
It is possible to implement `setof/3` with a single `sort/2`. But for the moment, correctness has priority.