return -1;
    }
    ListIterator<?> itr = list.listIterator();
    int computedIndex = 0;
    while (itr.hasNext()) {
      int axisIndex = itr.nextIndex();
      int elemIndex = axes.get(axisIndex).indexOf(itr.next());
      if (elemIndex == -1) {
        return -1;
      }
      computedIndex += elemIndex * axesSizeProduct[axisIndex + 1];
    }
    return computedIndex;
  }

  @Override

    Iterator<T> iteratorA = iterableA.iterator();
    Iterator<T> iteratorB = iterableB.iterator();

    while (iteratorA.hasNext() && iteratorB.hasNext()) {
      if (!elementEquivalence.equivalent(iteratorA.next(), iteratorB.next())) {
        return false;
      }
    }

    return !iteratorA.hasNext() && !iteratorB.hasNext();
  }

  @Override
  protected int doHash(Iterable<T> iterable) {
    int hash = 78721;

        }

        return type.cast(object);
    }

    /**
     * Gets the next element from an enumeration and casts it to the specified type
     * @param <T> the target type
     * @param type the target class type
     * @param enumeration the enumeration to read from
     * @return next element from enumeration cast to type
     * @throws PACDecodingException if types are incompatible
     */

    this.it = readers;
    advance();
  }

  /** Closes the current reader and opens the next one, if any. */
  private void advance() throws IOException {
    close();
    if (it.hasNext()) {
      current = it.next().openStream();
    }
  }

  @Override
  public int read(char[] cbuf, int off, int len) throws IOException {
    checkNotNull(cbuf);

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

  @CanIgnoreReturnValue
  @Override
  @ParametricNullness
  public T next() {
    return delegate().next();
  }

  @Override
  public void remove() {
    delegate().remove();
  }

    assertEquals(mapEntry(k0(), v0()), iterator.next());
    iterator.remove();
    assertTrue(multimap().isEmpty());
  }

  @CollectionSize.Require(absent = ZERO)
  @MapFeature.Require(SUPPORTS_REMOVE)
  public void testEntriesRemainValidAfterRemove() {
    Iterator<Entry<K, V>> iterator = multimap().entries().iterator();
    Entry<K, V> entry = iterator.next();
    K key = entry.getKey();

    assertThrows(
        ConcurrentModificationException.class,
        () -> {
          Iterator<E> iterator = collection.iterator();
          assertTrue(collection.remove(e0()));
          iterator.next();
        });
  }

  @CollectionFeature.Require(SUPPORTS_REMOVE)
  public void testRemove_notPresent() {
    assertFalse("remove(notPresent) should return false", collection.remove(e3()));
    expectUnchanged();

): List<T> {
  val ia = a.iterator()
  val ib = b.iterator()

  return buildList {
    while (ia.hasNext() || ib.hasNext()) {
      if (ia.hasNext()) {
        add(ia.next())
      }
      if (ib.hasNext()) {
        add(ib.next())
      }
    }
  }

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 { #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/"

    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(