code: public static PublishUpdate createPublishUpdate(Identifier i1, Identifier i2,
  			Document md, MetadataLifetime lifetime) {
  		if (md == null) {
  			throw new NullPointerException("md not allowed to be null");
  		}
  
  		List<Document> list = new ArrayList<Document>(1);
  		list.add(md);
  
  		return createPublishUpdate(i1, i2, list, lifetime);
  	}
docs: Create a new {@link PublishUpdate} instance that is used to publish
  metadata on a link between two {@link Identifier} instances with a specific
  {@link MetadataLifetime}.
  
  @param i1 the first {@link Identifier} of the link
  @param i2 the second {@link Identifier} of the link
  @param md the metadata that shall be published
  @param lifetime the lifetime of the new metadata
  @return the new {@link PublishUpdate} instance