checkNotNull(appendable);
    if (parts.hasNext()) {
      appendable.append(toString(parts.next()));
      while (parts.hasNext()) {
        appendable.append(separator);
        appendable.append(toString(parts.next()));
      }
    }
    return appendable;
  }

  /**

    LinearCongruentialGenerator generator = new LinearCongruentialGenerator(input);
    int candidate = 0;
    int next;

    // Jump from bucket to bucket until we go out of range
    while (true) {
      next = (int) ((candidate + 1) / generator.nextDouble());
      if (next >= 0 && next < buckets) {
        candidate = next;
      } else {
        return candidate;
      }
    }
  }

  /**

   *       next character.
   *       <ol>
   *         <li><b>If the end of the sequence was reached, the negated value of the trailing high
   *             surrogate is returned.</b>
   *         <li><b>If the next character was a valid low surrogate, the code point value of the
   *             high/low surrogate pair is returned.</b>
   *         <li>If the next character was not a low surrogate value, then {@link

): 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/"

   * }
   *
   * <p><b>Note:</b> This method <i>may</i> work with certain non-ASCII text but is not safe for use
   * with arbitrary Unicode text. It is mostly intended for use with text that is known to be safe
   * for use with it (such as all-ASCII text) and for simple debugging text. When using this method,
   * consider the following:
   *
   * <ul>
   *   <li>it may split surrogate pairs

  // http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/main/java/util/concurrent/

  /** The queued items */
  final E[] items;

  /** items index for next take, poll or remove */
  int takeIndex;

  /** items index for next put, offer, or add. */
  int putIndex;

  /** Number of items in the queue */
  private int count;

  /*
   * Concurrency control uses the classic two-condition algorithm

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

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

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

  @Override

	nameAddr := p.address(operands[0])
	if !p.validSymbol("TEXT", &nameAddr, false) {
		return
	}
	name := symbolName(&nameAddr)
	next := 1

	// Next operand is the optional text flag, a literal integer.
	var flag = int64(0)
	if len(operands) == 3 {
		flag = p.evalInteger("TEXT", operands[1])
		next++
	}

	// Issue an error if we see a function defined as ABIInternal

    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();