@Override
      @ParametricNullness
      public T next() {
        T next = iterator.next();
        iterator.remove();
        return next;
      }

      @Override
      public String toString() {
        return "Iterators.consumingIterator(...)";
      }
    };
  }

  /**
   * Deletes and returns the next value from the iterator, or returns {@code null} if there is no

    assertTrue(cycle.hasNext());
    assertEquals("a", cycle.next());
    cycle.remove();
    assertEquals(Collections.singletonList("b"), iterable);
    assertTrue(cycle.hasNext());
    assertEquals("b", cycle.next());
    assertTrue(cycle.hasNext());
    assertEquals("b", cycle.next());
    cycle.remove();
    assertEquals(Collections.emptyList(), iterable);
    assertFalse(cycle.hasNext());
  }

    checkNotNull(comparator);
    Iterator<? extends T> it = iterable.iterator();
    if (it.hasNext()) {
      T prev = it.next();
      while (it.hasNext()) {
        T next = it.next();
        if (comparator.compare(prev, next) > 0) {
          return false;
        }
        prev = next;
      }
    }
    return true;
  }

  /**

import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.collect.SortedLists.KeyAbsentBehavior.NEXT_HIGHER;
import static com.google.common.collect.SortedLists.KeyAbsentBehavior.NEXT_LOWER;
import static com.google.common.collect.SortedLists.KeyPresentBehavior.ANY_PRESENT;
import static java.util.Objects.requireNonNull;

import com.google.common.annotations.Beta;

     * counts are converted to strings as by {@code String.valueOf}.
     */
    @Override
    public String toString() {
      String text = String.valueOf(getElement());
      int n = getCount();
      return (n == 1) ? text : (text + " x " + n);
    }
  }

  /** An implementation of {@link Multiset#equals}. */
  static boolean equalsImpl(Multiset<?> multiset, @CheckForNull Object object) {

					e := f.updateCache.find(parent.Key())
					if e == nil || e.Compacted {
						foundAny = true
						break
					}
					next := f.updateCache.searchParent(parent)
					if next == nil {
						foundAny = true
						break
					}
					parent = *next
				}
				if !foundAny {
					// Add non-compacted empty entry.
					f.updateCache.replaceHashed(h, &thisHash, dataUsageEntry{})
				}
			}

I'll show you some concrete examples in the next chapters.

---

Then the next concepts to consider are all about the program running your actual API (e.g. Uvicorn).

## Program and Process

            range.lowerBound,
            KeyPresentBehavior.FIRST_AFTER,
            KeyAbsentBehavior.NEXT_HIGHER);
    int upperIndex =
        SortedLists.binarySearch(
            ranges,
            Range::lowerBound,
            range.upperBound,
            KeyPresentBehavior.ANY_PRESENT,
            KeyAbsentBehavior.NEXT_HIGHER);
    if (lowerIndex >= upperIndex) {
      return ImmutableRangeMap.of();
    }

          @Override
          @CheckForNull
          protected E computeNext() {
            if (itr1.hasNext()) {
              return itr1.next();
            }
            while (itr2.hasNext()) {
              E e = itr2.next();
              if (!set1.contains(e)) {
                return e;
              }
            }
            return endOfData();
          }
        };

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/"