return Collections2.safeContains(collection, element);
    }
    return Iterators.contains(iterable.iterator(), element);
  }

  /**
   * Removes, from an iterable, every element that belongs to the provided collection.
   *
   * <p>This method calls {@link Collection#removeAll} if {@code iterable} is a collection, and
   * {@link Iterators#removeAll} otherwise.
   *

/**
 * A support class for {@code ListenableFuture} implementations to manage their listeners. An
 * instance contains a list of listeners, each with an associated {@code Executor}, and guarantees
 * that every {@code Runnable} that is {@linkplain #add added} will be executed after {@link
 * #execute()} is called. Any {@code Runnable} added after the call to {@code execute} is still

   *
   * <p>The returned set isn't threadsafe or serializable, even if {@code unfiltered} is.
   *
   * <p>Many of the filtered set's methods, such as {@code size()}, iterate across every element in
   * the underlying set and determine which elements satisfy the filter. When a live view is
   * <i>not</i> needed, it may be faster to copy {@code Iterables.filter(unfiltered, predicate)} and
   * use the copy.

   * underlying multimap.
   *
   * <p>The returned multimap isn't threadsafe or serializable, even if {@code unfiltered} is.
   *
   * <p>Many of the filtered multimap's methods, such as {@code size()}, iterate across every
   * key/value mapping in the underlying multimap and determine which satisfy the filter. When a
   * live view is <i>not</i> needed, it may be faster to copy the filtered multimap and use the
   * copy.
   *

   *
   * <p>The returned multiset isn't threadsafe or serializable, even if {@code unfiltered} is.
   *
   * <p>Many of the filtered multiset's methods, such as {@code size()}, iterate across every
   * element in the underlying multiset and determine which elements satisfy the filter. When a live
   * view is <i>not</i> needed, it may be faster to copy the returned multiset and use the copy.
   *

              // this has 2 benefits
              // 1. for long chains of futures strung together with setFuture we consume less stack
              // 2. we avoid allocating Cancellation objects at every level of the cancellation
              //    chain
              // We can only do this for TrustedFuture, because TrustedFuture.cancel is final and
              // does nothing but delegate to this method.

 * environment if possible) before spending a lot of effort on tweaking a particular element.
 *
 * <h3>Other types of preconditions</h3>
 *
 * <p>Not every type of precondition failure is supported by these methods. Continue to throw
 * standard JDK exceptions such as {@link java.util.NoSuchElementException} or {@link
 * UnsupportedOperationException} in the situations they are intended for.

   * map.
   *
   * <p>The returned map isn't threadsafe or serializable, even if {@code unfiltered} is.
   *
   * <p>Many of the filtered map's methods, such as {@code size()}, iterate across every key/value
   * mapping in the underlying map and determine which satisfy the filter. When a live view is
   * <i>not</i> needed, it may be faster to copy the filtered map and use the copy.
   *

      while (iterator.hasNext()) {
        if (element.equals(iterator.next())) {
          return true;
        }
      }
    }
    return false;
  }

  /**
   * Traverses an iterator and removes every element that belongs to the provided collection. The
   * iterator will be left exhausted: its {@code hasNext()} method will return {@code false}.
   *
   * @param removeFrom the iterator to (potentially) remove elements from

    /*
     * We use `fromNullable` instead of `of` because `WeakReference.get()` has a nullable return
     * type.
     *
     * In practice, we are very unlikely to see `null`: The `WeakReference` to the enum constant
     * won't be cleared as long as the enum constant is referenced somewhere, and the enum constant