private ModifierKeyword modifierFor(ModifierKeyword foundModifier) {
    int index = VISIBILITY_MODS.indexOf(foundModifier);
    index += direction;
    if (index < -1) {
      index = VISIBILITY_MODS.size() - 1;
    } else if (index >= VISIBILITY_MODS.size()) {
      index = -1;
    }
    if (index == -1) {
      return null;
    }
    return VISIBILITY_MODS.get(index);
  }