code: public CompletionStage<Void> put(final Resource resource, final Instant time) {
          return runAsync(() -> {
              final File resourceDir = FileUtils.getResourceDirectory(directory, resource.getIdentifier());
              if (!resourceDir.exists()) {
                  resourceDir.mkdirs();
              }
              FileUtils.writeMemento(resourceDir, resource, time.truncatedTo(SECONDS));
          });
      }
docs: Create a Memento from a resource at a particular time.
  @param resource the resource
  @param time the time to which the Memento corresponds
  @return the completion stage representing that the operation has completed