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 * <p>No cascading checks are performed against the return values of methods unless the method is a
 * static factory method. Neither are semantics of mutation methods such as {@code
 * someList.add(obj)} checked. For more detailed discussion of supported and unsupported cases, see
 * {@link #testEquals}, {@link #testNulls} and {@link #testSerializable}.
 *
 * <p>For testing against the returned instances from a static factory class, such as
 *
 * <pre>

  //    the initial write (just stronger guarantees if it does).
  //
  // See: http://cs.oswego.edu/pipermail/concurrency-interest/2015-January/013800.html
  // For a (long) discussion about this specific issue and the general futility of life.
  //
  // For the time being we are OK with the problem discussed above since it requires a caller to

   * logged.
   *
   * <p>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
   * the discussion in the {@link ListenableFuture#addListener ListenableFuture.addListener}
   * documentation.
   *
   * @param listener the listener to run when the manager changes state

      return this;
    }

    @CanIgnoreReturnValue
    Builder<E> combine(Builder<E> other) {
      requireNonNull(impl);
      requireNonNull(other.impl);
      /*
       * For discussion of requireNonNull, see the comment on the field.
       *
       * (And I don't believe there's any situation in which we call x.combine(y) when x is a plain