* Returns the positional index of the left edge of a tree <i>t</i>
* within a given root, as defined by the size of the yield of all
* material preceding <i>t</i>.
*/
public static int leftEdge(Tree t, Tree root) {
MutableInteger i = new MutableInteger(0);
if (leftEdge(t, root, i)) {
return i.intValue();
} else {
throw new RuntimeException("Tree is not a descendant of root.");
// return -1;
}
}