@Override
        public boolean hasNext() {
          return backingEntries.hasNext();
        }

        @Override
        public Multiset.Entry<E> next() {
          final Map.Entry<E, Integer> mapEntry = backingEntries.next();
          return new Multisets.AbstractEntry<E>() {
            @Override
            public E getElement() {
              return mapEntry.getKey();
            }

    assertThrows(
        ConcurrentModificationException.class,
        () -> {
          Iterator<Entry<K, V>> iterator = getMap().entrySet().iterator();
          put(e3());
          iterator.next();
        });
  }

  @MapFeature.Require({FAILS_FAST_ON_CONCURRENT_MODIFICATION, SUPPORTS_PUT})
  @CollectionSize.Require(absent = ZERO)
  public void testPutAbsentConcurrentWithKeySetIteration() {
    assertThrows(

				b.RunParallel(func(pb *testing.PB) {
					rng := rand.New(rand.NewSource(time.Now().UnixNano()))
					n := 0
					var latency int64
					managers := grid.Managers
					hosts := grid.Hosts
					for pb.Next() {
						// Pick a random manager.
						src, dst := rng.Intn(len(managers)), rng.Intn(len(managers))
						if src == dst {
							dst = (dst + 1) % len(managers)
						}
						local := managers[src]

                if ( sort == 0 ) {
                    if ( compareNames(dirents[ i ], name, iter.next()) < 0 ) {
                        break;
                    }
                }
                else if ( sort == 1 ) {
                    if ( compareSizes(dirents[ i ], name, iter.next()) < 0 ) {
                        break;
                    }
                }

        }

        return false
      }

      override fun next(): String {
        if (!hasNext()) throw NoSuchElementException()
        val result = nextUrl!!
        nextUrl = null
        canRemove = true
        return result
      }

      override fun remove() {
        check(canRemove) { "remove() before next()" }
        delegate.remove()
      }
    }
  }

      @Override
      public boolean hasNext() {
        return entryItr.hasNext();
      }

      @Override
      public V next() {
        return entryItr.next().getValue();
      }
    };
  }

  @Override
  public boolean contains(@CheckForNull Object object) {
    return object != null && Iterators.contains(iterator(), object);
  }

  @Override

		mu := PaddedMutex{NewDRWMutex(ds, "")}
		for pb.Next() {
			mu.Lock(id, source)
			mu.Unlock(context.Background())
		}
	})
}

func benchmarkMutex(b *testing.B, slack, work bool) {
	b.ResetTimer()
	b.ReportAllocs()

	mu := NewDRWMutex(ds, "")
	if slack {
		b.SetParallelism(10)
	}
	b.RunParallel(func(pb *testing.PB) {
		foo := 0
		for pb.Next() {
			mu.Lock(id, source)

                return true;
            }
        }
        return false;
    }

    @Override
    public E next() {
        if (!hasNext()) {
            throw new NoSuchElementException();
        }
        return iterators[index].next();
    }

    @Override
    public void remove() {
        throw new ClUnsupportedOperationException("remove");
    }

///

The only new thing is the `callbacks=invoices_callback_router.routes` as an argument to the *path operation decorator*. We'll see what that is next.

## Documenting the callback

The actual callback code will depend heavily on your own API app.

And it will probably vary a lot from one app to the next.

It could be just one or two lines of code, like:

```Python
callback_url = "https://example.com/api/v1/invoices/events/"

    /**
     *
     */
    @Test
    public void test() {
        final SingleValueIterator<String> it = new SingleValueIterator<String>("Foo");
        assertThat(it.hasNext(), is(true));
        assertThat(it.next(), is("Foo"));
        assertThat(it.hasNext(), is(not(true)));
    }