seemed to rise like a stalk out of a sea of green leaves that lay
far below her.

  `What CAN all that green stuff be?' said Alice.  `And where
HAVE my shoulders got to?  And oh, my poor hands, how is it I
can't see you?'  She was moving them about as she spoke, but no
result seemed to follow, except a little shaking among the
distant green leaves.

             * exception in seenExceptions. When we complete secondFuture with the same exception,
             * we want for AggregateFuture to still detect that it's been previously seen.
             */
            secondFuture.setException(sameInstance);
          }
        },
        directExecutor());
    firstFuture.setException(sameInstance);

    try {

             * exception in seenExceptions. When we complete secondFuture with the same exception,
             * we want for AggregateFuture to still detect that it's been previously seen.
             */
            secondFuture.setException(sameInstance);
          }
        },
        directExecutor());
    firstFuture.setException(sameInstance);

    try {

seemed to rise like a stalk out of a sea of green leaves that lay
far below her.

  `What CAN all that green stuff be?' said Alice.  `And where
HAVE my shoulders got to?  And oh, my poor hands, how is it I
can't see you?'  She was moving them about as she spoke, but no
result seemed to follow, except a little shaking among the
distant green leaves.

   * Example usage:
   *
   * <pre>{@code
   * Color red = new Color("red", 255, 0, 0);
   * ...
   * ImmutableSet<Color> allColors = ImmutableSet.of(red, green, blue);
   *
   * ImmutableMap<String, Color> colorForName =
   *     uniqueIndex(allColors, c -> c.toString());
   * assertThat(colorForName).containsEntry("red", red);
   * }</pre>
   *