@NonNull
  @Override
  public IdpListJWS fetchIdpList(URI idpListUrl) {
    return idpListCache.computeIfAbsent(
        idpListUrl.toString(), k -> delegate.fetchIdpList(idpListUrl));
  }
@Test
  void fetchIdpList() {
    var uri = URI.create("https://example.com/idpList");

    var expected = new IdpListJWS(null, null);
    when(delegate.fetchIdpList(uri)).thenReturn(expected);

    // when
    var got = sut.fetchIdpList(uri);

    // then
    verify(delegate).fetchIdpList(uri);
    assertEquals(expected, got);
  }