# bug in the pq-trees library ?

For the following PQ-tree named pqtree:

```
('P', [{3, 5, 7}, {4, 7}, {1, 7}, {3}, {6}, ('Q', [('P', [{0, 1, 3, 4}, {0, 6}]), {0, 2, 5}, ('P', [{2}, {2, 6}])])])
```

The command set_contiguous(4) gives:

```
sage: pqtree.set_contiguous(4)
(1, True)
sage: pqtree
('P', [{3, 5, 7}, {1, 7}, {3}, {6}, ('Q', [('P', [{2}, {2, 6}]), {0, 2, 5}, ('P', [{0, 6}, {0, 1, 3, 4}]), {4, 7}])])
```

Observe that the subset including 4 are not necessarily contiguous in this PQ-tree! Repeating the command gives:

```
sage: pqtree.set_contiguous(4)
(1, True)
sage: pqtree
('P', [{3, 5, 7}, {1, 7}, {3}, {6}, ('Q', [('P', [{2}, {2, 6}]), {0, 2, 5}, {0, 6}, {0, 1, 3, 4}, {4, 7}])])
```

Now it is OK, but we need to repeat twice the command.

Could you please provide the commands to build your pq-tree?

The initial PQ-tree is created like this :

Then, if we execute the command pqtree.set_contiguous() successively on 0, 2, 4 we obtain the outcome as mentionned before.

But we obtain a good outcome with this order 2,4,0 for example.