code: void root(ProtoMember protoMember) {
      if (protoMember == null) throw new NullPointerException("protoMember == null");
      checkArgument(!identifierSet.excludes(protoMember));
      checkArgument(!types.contains(protoMember.type()));
      members.put(protoMember.type(), protoMember);
    }
docs: Marks {@code protoMember}, throwing if it is explicitly excluded, or if its enclosing type is
  also specifically included. This implicitly excludes other members of the same type.