debug!("old_bytes {:?}", old_bytes);

        debug!("up_context_idx = {:?}", up_context_idx);
        debug!("{:?}", diff.marker.get(&diff.pos_a[up_context_idx].pos));

                let offset = old_bytes - diff.pos_a[up_context_idx].pos;
                debug!("offset {:?} {:?}", pos.0, offset);

                let mut offset = old_bytes - diff.pos_a[up_context_idx].pos;
                // Here, in the case where one side of the conflict
                // ended with a "last line without a `\n`", the offset
                // might be off by one.
                //
                // We detect that case by testing whether the vertex
                // length is the same as the length of the "line".
                let v_end = diff.pos_a[up_context_idx].vertex.end.0.into();
                if pos.0 + offset <= v_end {
                    //
                } else if pos.0 + offset == v_end + 1 {
                    assert_eq!(diff.contents_a[old_bytes - 1], b'\n');
                    offset -= 1
                } else {
                    panic!("{:?} {:?} {:?}", pos.0, offset, v_end);
                }