@Test public void batchReturnsLessThanInput() {

  Funnel.Batch<Integer, String> toSpell = funnel.through(batch::send);
  toSpell.accept(1);
  toSpell.accept(2);
  when(batch.send(asList(1, 2))).thenReturn(asList("one"));
  assertThrows(IllegalStateException.class, funnel::run);
}