@Test public void testGroupingBy_withReducer_twoEntriesDifferentGroups() {

  BiStream<String, Integer> salaries = BiStream.of("Joe", 100, "Tom", 200);
  assertKeyValues(salaries.collect(groupingBy(s -> s.charAt(0), Integer::sum)))
      .containsExactly('J', 100, 'T', 200)
      .inOrder();
}