new Throwable("Failure occurred while trying to finish a future.") {
              @Override
              public synchronized Throwable fillInStackTrace() {
                return this; // no stack trace
              }
            });
    final Throwable exception;

    Failure(Throwable exception) {
      this.exception = checkNotNull(exception);
    }
  }

   * exception, every unchecked exception as an unchecked exception, and every error as an error. In
   * addition, the cause of any {@code ExecutionException} is wrapped in order to ensure that the
   * new stack trace matches that of the current thread.
   *
   * <p>Instances of {@code exceptionClass} are created by choosing an arbitrary public constructor

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    @CheckForNull private Iterator<? extends T> toRemove;

    /* The iterator currently returning elements. */
    private Iterator<? extends T> iterator;

    /*
     * We track the "meta iterators," the iterators-of-iterators, below.  Usually, topMetaIterator
     * is the only one in use, but if we encounter nested concatenations, we start a deque of

    @CheckForNull private Iterator<? extends T> toRemove;

    /* The iterator currently returning elements. */
    private Iterator<? extends T> iterator;

    /*
     * We track the "meta iterators," the iterators-of-iterators, below.  Usually, topMetaIterator
     * is the only one in use, but if we encounter nested concatenations, we start a deque of