@Test
public void shouldBuildListOfPersonsFromEveryExpense() throws Exception {

    final Expense firstExpense = mock(Expense.class);
    final Expense secondExpense = mock(Expense.class);
    final Nobt nobt = aNobt()
            .withExpenses(firstExpense, secondExpense)
            .build();
    nobt.getParticipatingPersons();
    verify(firstExpense).getParticipants();
    verify(secondExpense).getParticipants();
}