+<informaltable frame="none"><tgroup cols="2"><tbody><row>
+<entry><mediaobject><imageobject><imagedata fileref="fig4a.png"/></imageobject></mediaobject></entry>
+<entry><mediaobject><textobject><literallayout class="monospaced">
+ A
+</literallayout></textobject></mediaobject></entry>
+</row><row>
+<entry><mediaobject><imageobject><imagedata fileref="fig4b.png"/></imageobject></mediaobject></entry>
+<entry><mediaobject><textobject><literallayout class="monospaced">
+ O
+ / \
+ A B
+</literallayout></textobject></mediaobject></entry>
+</row><row>
+<entry><mediaobject><imageobject><imagedata fileref="fig4c.png"/></imageobject></mediaobject></entry>
+<entry><mediaobject><textobject><literallayout class="monospaced">
+ O
+ / \
+ A O
+ / \
+ B C
+</literallayout></textobject></mediaobject></entry>
+</row></tbody></tgroup></informaltable>