To keep track of the current object and subject of discussion.

§1. Inform is deliberately minimal when allowing the use of pronouns which carry meanings from one sentence to another. It is unclear exactly how natural language does this, and while some theories are more persuasive than others, all seem vulnerable to odd cases that they get "wrong". It's therefore hard to program a computer to understand "it" so that human users are happy with the result.

But we try, just a little, by keeping track of the subject and object under discussion. Even this is tricky. Consider:

The Pavilion is a room. East is the Cricket Square.

East of where? Clearly of the current subject, the Pavilion (not the room kind). On the other hand,

On the desk is a pencil. It has description "2B."

"It" here is the pencil, not the desk. To disentangle such things, we keep track of two different running references: the current subject and the current object. English is an SVO language, so that in assertions of the form "X is Y", X is the subject and Y the object. But it will turn out to be more complicated than that, because we disregard all references which are not to tangible things and kinds.

inference_subject *Anaphora::get_current_subject?Usage of Anaphora::get_current_subject:
Assertions - §2, §6.3.41.1, §6.3.41.4(void) {
    return global_pass_state.subject_of_sentences;
}

inference_subject *Anaphora::get_current_object?Usage of Anaphora::get_current_object:
Refine Parse Tree - §11.7(void) {
    return global_pass_state.object_of_sentences;
}

int Anaphora::get_current_subject_plurality?Usage of Anaphora::get_current_subject_plurality:
Refine Parse Tree - §11.7(void) {
    return global_pass_state.subject_seems_to_be_plural;
}

§2. The routine Anaphora::new_discussion is called when we reach a heading or other barrier in the source text, to make clear that there has been a change of the topic discussed.

Anaphora::change_discussion_topic is called once at the end of processing each assertion during each pass.

Note that we are careful to avoid changing the subject with sentences like:

East is the Central Plaza.

where this does not have the subject "east", but has instead an implicit subject carried over from previous sentences.

void Anaphora::new_discussion?Usage of Anaphora::new_discussion:
§3
Passes through Major Nodes - §2, §3.1(void) {
    if (global_pass_state.subject_of_sentences)
        LOGIF(PRONOUNS, "[Forgotten subject of sentences: $j]\n",
            global_pass_state.subject_of_sentences);
    if (global_pass_state.subject_of_sentences)
        LOGIF(PRONOUNS, "[Forgotten object of sentences: $j]\n",
            global_pass_state.object_of_sentences);
    global_pass_state.subject_of_sentences = NULL;
    global_pass_state.object_of_sentences = NULL;
    global_pass_state.subject_seems_to_be_plural = FALSE;
}

§3. The slight asymmetry in what follows is partly pragmatic, partly the result of subject-verb inversion ("in the bag is the ball" not "the ball is in the bag"). We extract a subject from a relationship node on the left, but not on the right, and we don't extract an object from one. Consider:

A billiards table is in the Gazebo. On it is a trophy cup.

What does "it" mean, and why? A human reader goes for the billiards table at once, because it seems more likely as a supporter than the Gazebo, but that's not how Inform gets the same answer. It all hangs on "billiards table" being the object of the first sentence, not the Gazebo; if we descended the RHS, which is RELATIONSHIP_NT -> PROPER_NOUN_NT pointing to the Gazebo, that's the conclusion we would have reached.

void Anaphora::change_discussion_from_coupling?Usage of Anaphora::change_discussion_from_coupling:
Passes through Major Nodes - §3.1.2.3(parse_node *px, parse_node *py) {
    inference_subject *infsx = NULL, *infsy = NULL, *infsy_full = NULL;
    infsx = Anaphora::discussed_at_node(px);
    infsy_full = Anaphora::discussed_at_node(py);
    if (Node::get_type(py) != KIND_NT) infsy = Node::get_subject(py);
    Anaphora::change_discussion_topic(infsx, infsy, infsy_full);
    if (Node::get_type(px) == AND_NT) Anaphora::subject_of_discussion_a_list();
    if (Annotations::read_int(current_sentence, clears_pronouns_ANNOT))
        Anaphora::new_discussion();
}

void Anaphora::change_discussion_topic(inference_subject *infsx,
    inference_subject *infsy, inference_subject *infsy_full) {
    inference_subject *old_sub = global_pass_state.subject_of_sentences,
        *old_obj = global_pass_state.object_of_sentences;
    global_pass_state.subject_seems_to_be_plural = FALSE;
    if (Wordings::length(Node::get_text(current_sentence)) > 1)
        global_pass_state.near_start_of_extension = 0;
    Node::set_interpretation_of_subject(current_sentence,
        global_pass_state.subject_of_sentences);

    if (Annotations::node_has(current_sentence, implicit_in_creation_of_ANNOT))
        return;
    #ifdef IF_MODULE
    if ((Map::subject_is_a_direction(infsx)) &&
            ((InstanceSubjects::to_object_instance(infsx) == NULL) ||
                (InstanceSubjects::to_object_instance(infsy_full)))) infsx = NULL;
    #endif
    if (infsx) global_pass_state.subject_of_sentences = infsx;
    if ((infsy) && (KindSubjects::to_kind(infsy) == NULL))
        global_pass_state.object_of_sentences = infsy;
    else if (infsx) global_pass_state.object_of_sentences = infsx;

    if (global_pass_state.subject_of_sentences != old_sub)
        LOGIF(PRONOUNS, "[Changed subject of sentences to $j]\n",
            global_pass_state.subject_of_sentences);
    if (global_pass_state.object_of_sentences != old_obj)
        LOGIF(PRONOUNS, "[Changed object of sentences to $j]\n",
            global_pass_state.object_of_sentences);
}

§4.

inference_subject *Anaphora::discussed_at_node?Usage of Anaphora::discussed_at_node:
§3(parse_node *pn) {
    inference_subject *infs = NULL;
    if (Node::get_type(pn) != KIND_NT) infs = Node::get_subject(pn);
    if ((Node::get_type(pn) == RELATIONSHIP_NT) && (pn->down) &&
        (Node::get_type(pn->down) == PROPER_NOUN_NT))
        infs = Node::get_subject(pn->down);
    if ((Node::get_type(pn) == WITH_NT) && (pn->down) &&
        (Node::get_type(pn->down) == PROPER_NOUN_NT))
        infs = Node::get_subject(pn->down);
    return infs;
}

§5. Occasionally we need to force the issue, though:

void Anaphora::subject_of_discussion_a_list?Usage of Anaphora::subject_of_discussion_a_list:
§3(void) {
    global_pass_state.subject_seems_to_be_plural = TRUE;
}