Re: [igraph] making sense of is_separator()

[Tamas Nepusz]

> I actually remember thinking a lot about this and that it was a
> deliberate choice to do it the way it was done.
>
> But I would need more time to dig out the reasons. Hopefully there are
> some comments somewhere in the code.

The problem with the current implementation is that it is inconsistent
with is_connected(). In my opinion, it should hold that is_separator()
returns true iff is_connected() returns false after the removal, given
that is_connected() was true before the removal. Since is_connected()
returns true for all graphs with a single vertex or no vertices, this
means that is_separator() must return false if there are |V|-1 or |V|
unique vertices in the vertex set to be removed.

If we happen to revert the changes that I've made in my most recent
commit, we must also change is_connected() to be consistent.

T.