static Map<S, O> simplifyWordTree(Map<S, O> tree) {
  if (tree == null) null;
  ret (Map) simplifyWordTree_impl(tree, true);
}

static O simplifyWordTree_impl(O tree, bool keepAsMap) {
  if (tree instanceof S) ret tree;
  Map<S, O> m = cast tree;
  if (!keepAsMap && l(m) == 1 && eq(firstKey(m), "<default>"))
    ret (S) firstValue(m);
  Map<S, O> map = similarEmptyMap(m);
  for (S key, O value : m) {
    map.put(key, simplifyWordTree_impl(value, false));
  }
  ret map;
}