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, WebStructure::section_language(S), identifier, TRUE);
                    break;
                case -1:
                    Str::clear(identifier);
                    IfdefTags::conditional_identifier(identifier, pt->the_tag);
                    LanguageMethods::open_ifdef(OUT, WebStructure::section_language(S), 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, WebStructure::section_language(S), identifier, TRUE);
                    break;
                case -1:
                    Str::clear(identifier);
                    IfdefTags::conditional_identifier(identifier, pt->the_tag);
                    LanguageMethods::close_ifdef(OUT, WebStructure::section_language(S), identifier, FALSE);
                    break;
            }
        }
        DISCARD_TEXT(identifier)
    }
}

§4. Effect on weaving. This provides the text for an endnote to a paragraph affected by conditional compilation; for a paragraph not thus affected, it writes nothing.

void IfdefTags::show_endnote_on_ifdefs(OUTPUT_STREAM, ls_paragraph *par) {
    int d = 0, sense = 1;
    Show ifdef endnoting4.1;
    sense = -1;
    Show ifdef endnoting4.1;
    if (d > 0) WRITE(".");
}

§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) {
                        WRITE("This paragraph is used only if ");
                    } else {
                        WRITE(" and if ");
                    }
                } else {
                    WRITE(" and ");
                }
                WRITE("%S", identifier);
            }
        if (c > 0) {
            if (c == 1) WRITE(" is");
            else WRITE(" are");
            if (sense == 1) WRITE(" defined");
            else WRITE(" undefined");
        }
    }