checkNotNull(listener, "Runnable was null.");
    checkNotNull(executor, "Executor was null.");
    // Checking isDone and listeners != TOMBSTONE may seem redundant, but our contract for
    // addListener says that listeners execute 'immediate' if the future isDone(). However, our
    // protocol for completing a future is to assign the value field (which sets isDone to true) and

    checkNotNull(listener, "Runnable was null.");
    checkNotNull(executor, "Executor was null.");
    // Checking isDone and listeners != TOMBSTONE may seem redundant, but our contract for
    // addListener says that listeners execute 'immediate' if the future isDone(). However, our
    // protocol for completing a future is to assign the value field (which sets isDone to true) and

  }

  /**
   * Returns a view of a multimap where each value is transformed by a function. All other
   * properties of the multimap, such as iteration order, are left intact. For example, the code:
   *
   * <pre>{@code
   * Multimap<String, Integer> multimap =
   *     ImmutableSetMultimap.of("a", 2, "b", -3, "b", -3, "a", 4, "c", 6);