Trac #16741: Bug in IncidenceStructure.automorphism_group when the ground set consists of pairs

Before

{{{
sage: designs.IncidenceStructure([[(1,2),(3,4)]]).automorphism_group()
...
ValueError: Invalid permutation vector: [(3, 4), (1, 2)]
}}}

After

{{{
sage: designs.IncidenceStructure([[(1,2),(3,4)]]).automorphism_group()
Permutation Group with generators [((1,2),(3,4))]
}}}

URL: http://trac.sagemath.org/16741
Reported by: ncohen
Ticket author(s): Nathann Cohen
Reviewer(s): Dima Pasechnik
Trac #15181: Treewidth (lazy implementation)

This code is a veeeery lazy implementation of a very lazy algorithm to
compute the treewidth of a graph. It can be greatly improved in at least
two places.

If it is to be improved later, this implementation will probably remain
useful anyway, as we try to always keep many of them alive, just to
check whether they all return sound results. Hence it's not so bad if it
is a bit wasteful `:-P`

And well. It's great to have this, too `:-P`

Nathann

URL: http://trac.sagemath.org/15181
Reported by: ncohen
Ticket author(s): Nathann Cohen
Reviewer(s): David Coudert