docs: private void merge(FieldNumberTree other) {
      for (Key key : other.children.keySet()) {
        FieldNumberTree value = other.children.get(key);
        if (!this.children.containsKey(key)) {
          this.children.put(key, value);
        } else {
          this.children.get(key).merge(value);
        }
      }
    }