To write the Extras element (Xt) in the index.

§1. This is to sweep up rulebooks and activities not covered by other elements, really, and most of the code here is just to arrange them in some logical order. 

void ExtrasElement::render(OUTPUT_STREAM, index_session *session) {
    localisation_dictionary *LD = Indexing::get_localisation(session);
    inter_tree *I = Indexing::get_tree(session);
    tree_inventory *inv = Indexing::get_inventory(session);
    InterNodeList::array_sort(inv->rulebook_nodes, MakeSynopticModuleStage::module_order);
    InterNodeList::array_sort(inv->activity_nodes, MakeSynopticModuleStage::module_order);

    inter_package *E;
    LOOP_OVER_INVENTORY_PACKAGES(E, i, inv->module_nodes)
        if (Metadata::read_optional_numeric(E, I"^category") == 1)
            Index rulebooks occurring in this part of the source text1.1;
    LOOP_OVER_INVENTORY_PACKAGES(E, i, inv->module_nodes)
        if (Metadata::read_optional_numeric(E, I"^category") == 2)
            Index rulebooks occurring in this part of the source text1.1;
}

§1.1. Index rulebooks occurring in this part of the source text1.1 = 

    int c = 0;
    inter_package *rb_pack;
    LOOP_OVER_INVENTORY_PACKAGES(rb_pack, i, inv->rulebook_nodes)
        if (MakeSynopticModuleStage::module_containing(rb_pack->package_head) == E) {
            if (Metadata::read_optional_numeric(rb_pack, I"^automatically_generated"))
                continue;
            if (c++ == 0) Heading for these1.1.1;
            IndexRules::rulebook_box(OUT, inv,
                Metadata::optional_textual(rb_pack, I"^printed_name"),
                NULL, rb_pack, NULL, 1, TRUE, session);
        }
    inter_package *av_pack;
    LOOP_OVER_INVENTORY_PACKAGES(av_pack, i, inv->activity_nodes)
        if (MakeSynopticModuleStage::module_containing(av_pack->package_head) == E) {
            if (c++ == 0) Heading for these1.1.1;
            IndexRules::activity_box(OUT, I, av_pack, 1, session);
        }

§1.1.1. Heading for these1.1.1 = 

    HTML_OPEN("p");
    WRITE("<b>");
    if (Metadata::read_optional_numeric(E, I"^category") == 1) {
        Localisation::roman(OUT, LD, I"Index.Elements.Xt.FromSourceText");
    } else {
        Localisation::roman_t(OUT, LD, I"Index.Elements.Xt.FromExtension",
            Metadata::optional_textual(E, I"^credit"));
    }
    WRITE("</b>");
    HTML_CLOSE("p");