static Map<S, O> simplifyWordTree(Map<S, O> tree) {
  if (tree == null) null;
  if (l(tree) == 1 && eq(firstKey(tree), "<default>"))
    ret (S) firstValue(tree);
  ret (Map) simplifyWordTree_impl(tree);
}

static O simplifyWordTree_impl(O tree) {
  if (tree instanceof S) ret tree;
  Map<S, O> m = cast tree;
  Map<S, O> map = similarEmptyMap(tm);
  for (S key, value : m)
    map.put(key, simplifyWordTree_impl(value));
  ret map;
}