in_neighbours() of undirected graphs are always empty

Hi Tiago,

I'm using graph-tool 2.2.36 with boost 1.57.0. I just found out that
in_neighbours() for undirected graphs are always empty. Is this intended or
a bug? Here's what I tried:

Thank you!

This is normal behavior inherited from the Boost graph
library: Undirected graphs have only out-neighbours and out-degrees.