code: void root(ProtoType type) {
      if (type == null) throw new NullPointerException("type == null");
      checkArgument(!identifierSet.excludes(type));
      checkArgument(!members.containsKey(type));
      types.add(type);
    }
docs: Marks {@code type}, throwing if it is explicitly excluded, or if any of its members are also
  specifically included.