chapter-11 subset problem

```
cd chapter-11 
swipl  practical-session.pl  
powerset([a,b,c,d,e],P).
P = [[], [a], [a, b], [a, b, c], [a, b, c, d], [a, b, c|...], [a, b|...], [a|...], [...|...]|...].
```

