you can get the indices of all the sites in the finialized system by using the `sites`
property of finalized systems. After that , it's just a case of writing a function that
can filter this list based on whether a site is on the `edge` or not. You would have
to define this filtering yourself. For graphene I would imagine that such a filter would
involve querying the number of neighbours of a particular site.

Happy Kwanting,