// 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
    // then to release waiters, followed by executing afterDone(), followed by releasing listeners.

 * <a href="http://www.unicode.org/glossary/#supplementary_code_point">supplementary Unicode code
 * points</a> in the range {@code 0x10000} to {@code 0x10FFFF} which includes the majority of
 * assigned characters, including important CJK characters and emoji.
 *
 * <p>Supplementary characters are <a
 * href="https://docs.oracle.com/javase/8/docs/api/java/lang/Character.html#supplementary">encoded

  // 'inputFuture'. That field is non-final and non-volatile. There are 2 places where the
  // 'inputFuture' field is read and where we will have to consider visibility of the write
  // operation in the constructor.
  //
  // 1. In the listener that performs the callback. In this case it is fine since inputFuture is

 * <a href="http://www.unicode.org/glossary/#supplementary_code_point">supplementary Unicode code
 * points</a> in the range {@code 0x10000} to {@code 0x10FFFF} which includes the majority of
 * assigned characters, including important CJK characters and emoji.
 *
 * <p>Supplementary characters are <a
 * href="https://docs.oracle.com/javase/8/docs/api/java/lang/Character.html#supplementary">encoded