let contents_res = {

                let contents = if contents.len() > 0 {

                    x

                    Some(Atom::NewVertex(x))
                } else {
                    None

                    contents: Some(Atom::NewVertex(contents_res)),

                    contents,

                        assert!(!contents.is_empty());

                // TODO: this code block looks suspect. Check the correctness.

                // If `contents.is_empty()`, we still need to add a
                // new empty vertex, so the following is ok:

                // TODO: this maths is probably unnecessarily complicated