// so that the cleanup work is visible to listeners.
      // afterDone() should be generally fast and only used for cleanup work... but in theory can
      // also be recursive and create StackOverflowErrors
      future.afterDone();
      // push the current set of listeners onto next
      next = future.clearListeners(next);
      future = null;

   *
   * <p>Example:
   *
   * <pre>{@code
   * static final ListMultimap<Character, String> FIRST_LETTER_MULTIMAP =
   *     Stream.of("banana", "apple", "carrot", "asparagus", "cherry")
   *         .collect(
   *             toMultimap(
   *                  str -> str.charAt(0),
   *                  str -> str.substring(1),
   *                  MultimapBuilder.treeKeys().arrayListValues()::build));