Thematic tags can be attached to certain paragraphs, some automatically by Inweb, others manually by the author.

§1. A tag really is just a textual name. Each differently-named tag leads to one of the following being created:

typedef struct theme_tag {
    struct text_stream *tag_name;
    int ifdef_positive;
    struct text_stream *ifdef_symbol;
    CLASS_DEFINITION
} theme_tag;

• The structure theme_tag is accessed in 5/htm, 8/ws, 1/ts, 2/tr, 4/is and here.

§2. Here we find a tag from its name, case-sensitively. On each run of Inweb, there's just a single namespace of all known tags. There are never very many differently-named tags in a given web.

theme_tag *Tags::find_by_name?Usage of Tags::find_by_name:
§4
Program Control - §7.4.3
The Weaver - §2.7.2.3.2.1, §3
Language Methods - §25(text_stream *name, int creating_if_necessary) {
    theme_tag *tag;
    LOOP_OVER(tag, theme_tag)
        if (Str::eq(name, tag->tag_name))
            return tag;
    if (creating_if_necessary) {
        tag = CREATE(theme_tag);
        tag->tag_name = Str::duplicate(name);
        tag->ifdef_positive = NOT_APPLICABLE;
        tag->ifdef_symbol = Str::new();
        if (Str::prefix_eq(name, I"ifdef-", 6)) {
            Str::substr(tag->ifdef_symbol, Str::at(name, 6), Str::end(name));
            tag->ifdef_positive = TRUE;
        } else if (Str::prefix_eq(name, I"ifndef-", 7)) {
            Str::substr(tag->ifdef_symbol, Str::at(name, 7), Str::end(name));
            tag->ifdef_positive = FALSE;
        }
        LanguageMethods::new_tag_declared(tag);
        return tag;
    }
    return NULL;
}

§3. A "tagging" occurs when a paragraph is marked with a given tag, and perhaps also with a contextually relevant caption. The following records those; they're stored as a linked list within each paragraph.

typedef struct paragraph_tagging {
    struct theme_tag *the_tag;
    struct text_stream *caption;
    CLASS_DEFINITION
} paragraph_tagging;

void Tags::add_to_paragraph?Usage of Tags::add_to_paragraph:
§4
The Parser - §1.1.3
InC Support - §3.1.3.5(paragraph *P, theme_tag *tag, text_stream *caption) {
    if (P) {
        paragraph_tagging *pt = CREATE(paragraph_tagging);
        pt->the_tag = tag;
        if (caption) pt->caption = Str::duplicate(caption);
        else pt->caption = Str::new();
        ADD_TO_LINKED_LIST(pt, paragraph_tagging, P->taggings);
    }
}

• The structure paragraph_tagging is accessed in 5/wt, 5/hf, 5/df and here.

§4. Tags are created simply by being used in taggings. If the tag notation ^"History: How tags came about" is found, the following is called, and the tag is History, the caption "How tags came about".

theme_tag *Tags::add_by_name?Usage of Tags::add_by_name:
The Reader - §4.4
The Parser - §1.1.2, §1.1.7.3, §1.1.7.7
Types and Functions - §2.2
C-Like Languages - §2.1
InC Support - §3.1.1.1(paragraph *P, text_stream *text) {
    if (Str::len(text) == 0) internal_error("empty tag name");
    TEMPORARY_TEXT(name) Str::copy(name, text);
    TEMPORARY_TEXT(caption)
    match_results mr = Regexp::create_mr();
    if (Regexp::match(&mr, name, U"(%c+?): (%c+)")) {
        Str::copy(name, mr.exp[0]);
        Str::copy(caption, mr.exp[1]);
    }
    theme_tag *tag = Tags::find_by_name(name, TRUE);
    if (P) Tags::add_to_paragraph(P, tag, caption);
    DISCARD_TEXT(name)
    DISCARD_TEXT(caption)
    Regexp::dispose_of(&mr);
    return tag;
}

§5. If a given line is tagged with a given tag, what caption does it have?

text_stream *Tags::retrieve_caption?Usage of Tags::retrieve_caption:
The Weaver - §2.7.2.1.1(paragraph *P, theme_tag *tag) {
    if (tag == NULL) return NULL;
    if (P) {
        paragraph_tagging *pt;
        LOOP_OVER_LINKED_LIST(pt, paragraph_tagging, P->taggings)
            if (tag == pt->the_tag)
                return pt->caption;
    }
    return NULL;
}

§6. Finally, this tests whether a given paragraph falls under a given tag. (Everything falls under the null non-tag: this ensures that a weave which doesn't specify a tag.)

int Tags::tagged_with?Usage of Tags::tagged_with:
The Weaver - §2.7, §2.7.2.3.2.1, §3
Language Methods - §25(paragraph *P, theme_tag *tag) {
    if (tag == NULL) return TRUE;
    if (P) {
        paragraph_tagging *pt;
        LOOP_OVER_LINKED_LIST(pt, paragraph_tagging, P->taggings)
            if (tag == pt->the_tag)
                return TRUE;
    }
    return FALSE;
}

§7.

void Tags::open_ifdefs?Usage of Tags::open_ifdefs:
The Tangler - §1.1.1.1, §2
C-Like Languages - §5, §6.1, §6.3, §6.4(OUTPUT_STREAM, paragraph *P) {
    paragraph_tagging *pt;
    LOOP_OVER_LINKED_LIST(pt, paragraph_tagging, P->taggings)
        if (Str::len(pt->the_tag->ifdef_symbol) > 0)
            LanguageMethods::open_ifdef(OUT,
                P->under_section->sect_language, pt->the_tag->ifdef_symbol, pt->the_tag->ifdef_positive);
}

void Tags::close_ifdefs?Usage of Tags::close_ifdefs:
The Tangler - §1.1.1.1, §2
C-Like Languages - §5, §6.1, §6.3, §6.4(OUTPUT_STREAM, paragraph *P) {
    paragraph_tagging *pt;
    LOOP_OVER_LINKED_LIST(pt, paragraph_tagging, P->taggings)
        if (Str::len(pt->the_tag->ifdef_symbol) > 0)
            LanguageMethods::close_ifdef(OUT,
                P->under_section->sect_language, pt->the_tag->ifdef_symbol, pt->the_tag->ifdef_positive);
}

void Tags::show_endnote_on_ifdefs?Usage of Tags::show_endnote_on_ifdefs:
The Weaver - §3(heterogeneous_tree *tree, tree_node *ap, paragraph *P) {
    int d = 0, sense = TRUE;
    Show ifdef endnoting7.1;
    sense = FALSE;
    Show ifdef endnoting7.1;
    if (d > 0) TextWeaver::commentary_text(tree, ap, I".");
}

§7.1. Show ifdef endnoting7.1 =

    int c = 0;
    paragraph_tagging *pt;
    LOOP_OVER_LINKED_LIST(pt, paragraph_tagging, P->taggings)
        if (pt->the_tag->ifdef_positive == sense)
            if (Str::len(pt->the_tag->ifdef_symbol) > 0) {
                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, pt->the_tag->ifdef_symbol);
            }
    if (c > 0) {
        if (c == 1) TextWeaver::commentary_text(tree, ap, I" is");
        else TextWeaver::commentary_text(tree, ap, I" are");
        if (sense) TextWeaver::commentary_text(tree, ap, I" defined");
        else TextWeaver::commentary_text(tree, ap, I" undefined");
    }

• This code is used in §7 (twice).