code: public static OPFItems build(Iterable<Builder> itemBuilders, Iterable<String> spineIDs)
    {
      return new OPFItems(Iterables.transform(Preconditions.checkNotNull(itemBuilders),
          new Function<OPFItem.Builder, OPFItem>()
          {
            @Override
            public OPFItem apply(Builder builder)
            {
              return builder.build();
            }
          }), Preconditions.checkNotNull(spineIDs));
    }
docs: Creates a consolidated set of {@link OPFItem} from item builders and a list
  of spine item IDs.
  
  @param itemBuilders
  the builders of the {@link OPFItem} in the set.
  @param spineIDs
  the IDs of the items in the spine.
  @return a consolidated set of {@link OPFItem}s.