Thematic tags can be used to mark code as being included in a tangle but only within markers for conditional compilation.

§1. Tag representation.Paragraph tagging is used to mark that code should be conditional. For example, a para which should be included only if WINDOWS is defined should be tagged ifdef-WINDOWS. The "valency" of that tag is 1, and the "identifier" is WINDOWS. 

int IfdefTags::ifdef_valency(text_stream *tag) {
    if (Str::begins_with(tag, I"ifdef-")) return 1;
    if (Str::begins_with(tag, I"ifndef-")) return -1;
    return 0;
}

void IfdefTags::conditional_identifier(OUTPUT_STREAM, text_stream *tag) {
    switch (IfdefTags::ifdef_valency(tag)) {
        case 1: Str::substr(OUT, Str::at(tag, 6), Str::end(tag)); break;
        case -1: Str::substr(OUT, Str::at(tag, 7), Str::end(tag)); break;
    }
}

§2. Effect on tangling.

void IfdefTags::open_ifdefs(OUTPUT_STREAM, ls_paragraph *par) {
    if ((par) && (par->taggings)) {
        TEMPORARY_TEXT(identifier)
        ls_section *S = LiterateSource::section_of_par(par);
        literate_source_tagging *pt;
        LOOP_OVER_LINKED_LIST(pt, literate_source_tagging, par->taggings) {
            switch (IfdefTags::ifdef_valency(pt->the_tag)) {
                case 1:
                    Str::clear(identifier);
                    IfdefTags::conditional_identifier(identifier, pt->the_tag);
                    LanguageMethods::open_ifdef(OUT, S->sect_language, identifier, TRUE);
                    break;
                case -1:
                    Str::clear(identifier);
                    IfdefTags::conditional_identifier(identifier, pt->the_tag);
                    LanguageMethods::open_ifdef(OUT, S->sect_language, identifier, FALSE);
                    break;
            }
        }
        DISCARD_TEXT(identifier)
    }
}

§3. This should arguably close the conditionals in reverse order, but it's a nuisance to loop backwards through a singly-linked list, and it doesn't really matter in practice. 

void IfdefTags::close_ifdefs(OUTPUT_STREAM, ls_paragraph *par) {
    if ((par) && (par->taggings)) {
        TEMPORARY_TEXT(identifier)
        ls_section *S = LiterateSource::section_of_par(par);
        literate_source_tagging *pt;
        LOOP_OVER_LINKED_LIST(pt, literate_source_tagging, par->taggings) {
            switch (IfdefTags::ifdef_valency(pt->the_tag)) {
                case 1:
                    Str::clear(identifier);
                    IfdefTags::conditional_identifier(identifier, pt->the_tag);
                    LanguageMethods::close_ifdef(OUT, S->sect_language, identifier, TRUE);
                    break;
                case -1:
                    Str::clear(identifier);
                    IfdefTags::conditional_identifier(identifier, pt->the_tag);
                    LanguageMethods::close_ifdef(OUT, S->sect_language, identifier, FALSE);
                    break;
            }
        }
        DISCARD_TEXT(identifier)
    }
}

§4. Effect on weaving.

void IfdefTags::show_endnote_on_ifdefs(heterogeneous_tree *tree, tree_node *ap, ls_paragraph *par) {
    int d = 0, sense = 1;
    Show ifdef endnoting4.1;
    sense = -1;
    Show ifdef endnoting4.1;
    if (d > 0) TextWeaver::commentary_text(tree, ap, I".");
}

§4.1. Show ifdef endnoting4.1 = 

    if ((par) && (par->taggings)) {
        int c = 0;
        literate_source_tagging *pt;
        LOOP_OVER_LINKED_LIST(pt, literate_source_tagging, par->taggings)
            if (IfdefTags::ifdef_valency(pt->the_tag) == sense) {
                TEMPORARY_TEXT(identifier)
                IfdefTags::conditional_identifier(identifier, pt->the_tag);
                if (c++ == 0) {
                    if (d++ == 0) {
                        tree_node *E = WeaveTree::endnote(tree);
                        Trees::make_child(E, ap); ap = E;
                        TextWeaver::commentary_text(tree, ap, I"This paragraph is used only if ");
                    } else {
                        TextWeaver::commentary_text(tree, ap, I" and if ");
                    }
                } else {
                    TextWeaver::commentary_text(tree, ap, I" and ");
                }
                TextWeaver::commentary_text(tree, ap, identifier);
            }
        if (c > 0) {
            if (c == 1) TextWeaver::commentary_text(tree, ap, I" is");
            else TextWeaver::commentary_text(tree, ap, I" are");
            if (sense == 1) TextWeaver::commentary_text(tree, ap, I" defined");
            else TextWeaver::commentary_text(tree, ap, I" undefined");
        }
    }