if (!(k1 instanceof Tree) || !(k2 instanceof Tree)) {
throw new IllegalArgumentException("svm_node does not contain tree data.");
}
Tree t1 = (Tree) k1;
Tree t2 = (Tree) k2;
List<Tree> nodes1 = getNodes(t1);
List<Tree> nodes2 = getNodes(t2);
int N1 = Math.min(MAX_NODE, nodes1.size());