code: public static PublishUpdate createPublishUpdate(Identifier i1, Identifier i2,
  			Collection<Document> mdlist, MetadataLifetime lifetime) {
  		if (mdlist == null) {
  			throw new NullPointerException("mdlist not allowed to be null");
  		}
  		PublishUpdate pu = createPublishUpdate();
  		fillMetadataHolder(pu, i1, i2, mdlist);
  		pu.setLifeTime(lifetime);
  		return pu;
  	}
docs: Create a new {@link PublishUpdate} instance that is used to publish
  a list of 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 mdlist a list of metadata objects
  @param lifetime the lifetime of the new metadata
  @return the new {@link PublishUpdate} instance