code: @Override
      public final boolean complete(Object value) {
          for (; ; ) {
              final Object oldState = state;
              if (isDone(oldState)) {
                  warnIfSuspiciousDoubleCompletion(oldState, value);
                  return false;
              }
              if (compareAndSetState(oldState, value)) {
                  onComplete();
                  unblockAll(oldState, defaultExecutor);
                  return true;
              }
          }
      }
docs: Can be called multiple times, but only the first answer will lead to the
  future getting triggered. All subsequent complete calls are ignored.
  
  @param value The type of response to offer.
  @return <tt>true</tt> if offered response, either a final response or an
  internal response, is set/applied, <tt>false</tt> otherwise. If <tt>false</tt>
  is returned, that means offered response is ignored because a final response
  is already set to this future.