ng whether it is symmetric or not, that is whether
<code>i</code> is a neighbour of <code>j</code> implies that
<code>j</code> is a neighbour of <code>i</code>. Some neighbour
definitions are symmetric by construction, such as contiguities or
distance thresholds, others are asymmetric, such as
<code>k</code>-nearest neighbours. The <code>nb</code> object
redundantly stores both <code>i</code>-<code>j</code> and
<code>j</code>-<code>i</code> links.</p>
<p>The second component of a <code>listw</code> object i