following problems: </p>
<ul>
<li><b>More than two facets meeting at a ridge. </b>When
Qhull creates facets, it creates an even number
of facets for each ridge. A convex hull always
has two facets for each ridge. More than two
facets may be created if non-adjacent facets
share a vertex. This is called a <em>duplicate
ridge</em>. In 2-d, a duplicate ridge would
create a loop of facets. </li>
</ul>
<ul>
<li><b>A facet contained in another facet. </b>Facet
merging may leave all vertices of one facet as a
subse