private Tree<String> merge(Tree<String> leftTree, Tree<String> rightTree) {
int span = leftTree.getYield().size() + rightTree.getYield().size();
String mostFrequentLabel = spanToCategories.getCounter(span).argMax();
List<Tree<String>> children = new ArrayList<Tree<String>>();
children.add(leftTree);
children.add(rightTree);
return new Tree<String>(mostFrequentLabel, children);
}
