 list between the entry
      specified by pointer value I, up to, but not including, the entry
      specified by pointer I+1.
    </p>

    <p>
      Since it is quite common to represent a graph as a drawing,
      or to abstract a graph from a physical situation in which the
      nodes have positions, it is also useful to have available the
      <b>node coordinates</b>, which are simply the (X,Y) coordinates
      of each node.
    </p>

    <p>
      Similarly, it may be more suggestive to refer to n