@Override
    public Optional<RawFlag> fetch(FlagId id, FetchVector vector) {
        return sources.stream()
                .map(source -> source.fetch(id, vector))
                .filter(Optional::isPresent)
                .map(Optional::get)
                .findFirst();
    }
@Test
    void test() {
        FlagSource source1 = mock(FlagSource.class);
        FlagSource source2 = mock(FlagSource.class);
        OrderedFlagSource orderedSource = new OrderedFlagSource(source1, source2);

        FlagId id = new FlagId("id");
        FetchVector vector = new FetchVector();

        when(source1.fetch(any(), any())).thenReturn(Optional.empty());
        when(source2.fetch(any(), any())).thenReturn(Optional.empty());
        assertFalse(orderedSource.fetch(id, vector).isPresent());
        verify(source1, times(1)).fetch(any(), any());
        verify(source2, times(1)).fetch(any(), any());

        RawFlag rawFlag = mock(RawFlag.class);

        when(source1.fetch(any(), any())).thenReturn(Optional.empty());
        when(source2.fetch(any(), any())).thenReturn(Optional.of(rawFlag));
        assertEquals(orderedSource.fetch(id, vector), Optional.of(rawFlag));
        verify(source1, times(2)).fetch(any(), any());
        verify(source2, times(2)).fetch(any(), any());

        when(source1.fetch(any(), any())).thenReturn(Optional.of(rawFlag));
        when(source2.fetch(any(), any())).thenReturn(Optional.empty());
        assertEquals(orderedSource.fetch(id, vector), Optional.of(rawFlag));
        verify(source1, times(3)).fetch(any(), any());
        // Not invoked as source1 provided raw flag
        verify(source2, times(2)).fetch(any(), any());
    }