// A note on memory visibility.
  // Many of the utilities in this class (transform, withFallback, withTimeout, asList, combine)
  // have two requirements that significantly complicate their design.
  // 1. Cancellation should propagate from the returned future to the input future(s).
  // 2. The returned futures shouldn't unnecessarily 'pin' their inputs after completion.
  //

 * API</h2>
 *
 * <p>The successor to Guava's caching API is <a
 * href="https://github.com/ben-manes/caffeine/wiki">Caffeine</a>. Its API is designed to make it a
 * nearly drop-in replacement. It requires Java 8+, and is not available for Android or GWT/J2CL,
 * and may have <a href="https://github.com/ben-manes/caffeine/wiki/Guava">different (usually

  // * Future completion is defined by when #value becomes non-null/non SetFuture
  // * Future completion can be observed if the waiters field contains a TOMBSTONE

  // Timed Get
  // There are a few design constraints to consider
  // * We want to be responsive to small timeouts, unpark() has non trivial latency overheads (I
  //   have observed 12 micros on 64-bit linux systems to wake up a parked thread). So if the