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

§1. 

void ExtrasElement::render(OUTPUT_STREAM, localisation_dictionary *LD) {
    inter_tree *I = InterpretIndex::get_tree();
    tree_inventory *inv = Synoptic::inv(I);
    TreeLists::sort(inv->rulebook_nodes, Synoptic::module_order);
    TreeLists::sort(inv->activity_nodes, Synoptic::module_order);

    for (int i=0; i<TreeLists::len(inv->module_nodes); i++) {
        inter_package *E = Inter::Package::defined_by_frame(inv->module_nodes->list[i].node);
        if (Metadata::read_optional_numeric(E, I"^category") == 1)
            Index rulebooks occurring in this part of the source text1.1;
    }
    for (int i=0; i<TreeLists::len(inv->module_nodes); i++) {
        inter_package *E = Inter::Package::defined_by_frame(inv->module_nodes->list[i].node);
        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;
    for (int j=0; j<TreeLists::len(inv->rulebook_nodes); j++) {
        if (Synoptic::module_containing(inv->rulebook_nodes->list[j].node) == E) {
            inter_package *pack =
                Inter::Package::defined_by_frame(inv->rulebook_nodes->list[j].node);
            if (Metadata::read_optional_numeric(pack, I"^automatically_generated"))
                continue;
            if (c++ == 0) Heading for these1.1.1;
            IndexRules::rulebook_box(OUT, inv,
                Metadata::read_optional_textual(pack, I"^printed_name"),
                NULL, pack, NULL, 1, TRUE, LD);
        }
    }
    for (int j=0; j<TreeLists::len(inv->activity_nodes); j++) {
        if (Synoptic::module_containing(inv->activity_nodes->list[j].node) == E) {
            inter_package *pack =
                Inter::Package::defined_by_frame(inv->activity_nodes->list[j].node);
            if (c++ == 0) Heading for these1.1.1;
            IndexRules::activity_box(OUT, I, pack, 1, LD);
        }
    }

§1.1.1. Heading for these1.1.1 = 

    HTML_OPEN("p");
    if (Metadata::read_optional_numeric(E, I"^category") == 1) {
        WRITE("<b>From the source text</b>");
    } else {
        WRITE("<b>From the extension %S</b>",
            Metadata::read_optional_textual(E, I"^credit"));
    }
    HTML_CLOSE("p");