first to find candidate
neighbours from intersecting polygons (<code><a href="https://r-spatial.github.io/sf/reference/geos_binary_pred.html" class="external-link">st_intersects()</a></code>),
and second to use the symmetry of the relationship to halve the number
of remaining tests. This means that performance is linear in n, but with
overhead for identifying candidates, and back-filling symmetric
neighbours. Further, <code><a href="../reference/poly2nb.html">spdep::poly2nb()</a></code> stops searching for
