By the time she had caught the flamingo and brought it back,
the fight was over, and both the hedgehogs were out of sight:
`but it doesn't matter much,' thought Alice, `as all the arches
are gone from this side of the ground.'  So she tucked it away
under her arm, that it might not escape again, and went back for
a little more conversation with her friend.

  When she got back to the Cheshire Cat, she was surprised to

    }
  }

  /** A reference to a value. */
  interface ValueReference<K, V> {
    /** Returns the value. Does not block or throw exceptions. */
    @CheckForNull
    V get();

    /**
     * Waits for a value that may still be loading. Unlike get(), this method can block (in the case
     * of FutureValueReference).
     *
     * @throws ExecutionException if the loading thread throws an exception

  By the time she had caught the flamingo and brought it back,
the fight was over, and both the hedgehogs were out of sight:
`but it doesn't matter much,' thought Alice, `as all the arches
are gone from this side of the ground.'  So she tucked it away
under her arm, that it might not escape again, and went back for
a little more conversation with her friend.

  When she got back to the Cheshire Cat, she was surprised to

 * "value" of a successful step or the "result" (value or exception) of any step.
 *
 * <ol>
 *   <li>A pipeline starts at its leaf step (or steps), which is created from either a callable
 *       block or a {@link ListenableFuture}.
 *   <li>Each other step is derived from one or more input steps. At each step, zero or more objects
 *       can be captured for later closing.

   *     HashBiMap.<Long, String>create());
   * ...
   * Set<Long> set = map.keySet();  // Needn't be in synchronized block
   * ...
   * synchronized (map) {  // Synchronizing on map, not set!
   *   Iterator<Long> it = set.iterator(); // Must be in synchronized block
   *   while (it.hasNext()) {
   *     foo(it.next());
   *   }
   * }
   * }</pre>
   *

    }
  }

  /** A reference to a value. */
  interface ValueReference<K, V> {
    /** Returns the value. Does not block or throw exceptions. */
    @CheckForNull
    V get();

    /**
     * Waits for a value that may still be loading. Unlike get(), this method can block (in the case
     * of FutureValueReference).
     *
     * @throws ExecutionException if the loading thread throws an exception