 *));
</B></PRE>Insert an item into an AVL tree, and return the updated tree.
The item is an arbitrary pointer, and the caller gives a
function that takes two pointers and compares two items.
The comparison function must return LESS_THAN, SAME_AS, or
GREATER_THAN.
<p>
If the item is already in the tree (that is, if the comparison
function return SAME_AS for any item already in the tree),
a fatal error occurs.
<A NAME="avl_item_at_position"></A><HR><PRE><B>void *avl_item_at_position(Avl_node p, double pos);
