To define that prince among predicates, the equality relation; and also its less noble sidekick, the "has" relation.

• §2. Family
• §3. Initial stock
• §4. Problem message text

§1. Equality is the only relation in its family; but in Inform, there will be other relations besides "has" in the spatial family.

bp_family *equality_bp_family = NULL;
bp_family *spatial_bp_family = NULL;
bp_family *dialogue_bp_family = NULL;
bp_family *empty_bp_family = NULL;

binary_predicate *R_equality = NULL;
binary_predicate *a_has_b_predicate = NULL;
binary_predicate *R_empty = NULL;

§2. Family. This is a minimal representation only, for when the calculus module is used in a non-Inform context: whereas Inform adds other methods to the equality family to handle its typechecking in The Equality Relation Revisited (in assertions).

void Calculus::Equality::start?Usage of Calculus::Equality::start:
Calculus Module - §3(void) {
    equality_bp_family = BinaryPredicateFamilies::new();
    METHOD_ADD(equality_bp_family, STOCK_BPF_MTID,
        Calculus::Equality::stock);
    METHOD_ADD(equality_bp_family, DESCRIBE_FOR_PROBLEMS_BPF_MTID,
        Calculus::Equality::describe_for_problems);
    METHOD_ADD(equality_bp_family, DESCRIBE_FOR_INDEX_BPF_MTID,
        Calculus::Equality::describe_for_index);

    spatial_bp_family = BinaryPredicateFamilies::new();
    #ifndef IF_MODULE
    METHOD_ADD(spatial_bp_family, STOCK_BPF_MTID,
        Calculus::Equality::stock_spatial);
    #endif

    dialogue_bp_family = BinaryPredicateFamilies::new();

    empty_bp_family = BinaryPredicateFamilies::new();
    METHOD_ADD(empty_bp_family, STOCK_BPF_MTID,
        Calculus::Equality::stock_empty);
    METHOD_ADD(empty_bp_family, DESCRIBE_FOR_PROBLEMS_BPF_MTID,
        Calculus::Equality::describe_empty_for_problems);
    METHOD_ADD(empty_bp_family, DESCRIBE_FOR_INDEX_BPF_MTID,
        Calculus::Equality::describe_empty_for_index);
}

§3. Initial stock. Note the unique one-off way in which equality is made.

void Calculus::Equality::stock?Usage of Calculus::Equality::stock:
§2(bp_family *self, int n) {
    if (n == 1) {
        R_equality = BinaryPredicates::make_equality(equality_bp_family,
            PreformUtilities::wording(<relation-names>, EQUALITY_RELATION_NAME));
        BinaryPredicates::set_index_details(R_equality, "value", "value");
    }
}

void Calculus::Equality::stock_spatial?Usage of Calculus::Equality::stock_spatial:
§2(bp_family *self, int n) {
    if (n == 1) {
        a_has_b_predicate =
            BinaryPredicates::make_pair(spatial_bp_family,
                BPTerms::new_full(NULL, NULL, EMPTY_WORDING, NULL),
                BPTerms::new(NULL),
                I"has", I"is-had-by",
                NULL, NULL,
                PreformUtilities::wording(<relation-names>, POSSESSION_RELATION_NAME));
    }
}

void Calculus::Equality::stock_empty?Usage of Calculus::Equality::stock_empty:
§2(bp_family *self, int n) {
    if (n == 1) {
        R_empty = BinaryPredicates::make_equality(empty_bp_family,
            PreformUtilities::wording(<relation-names>, EMPTY_RELATION_NAME));
        BinaryPredicates::set_index_details(R_equality, "value", "value");
    }
}

§4. Problem message text.

int Calculus::Equality::describe_for_problems?Usage of Calculus::Equality::describe_for_problems:
§2(bp_family *self, OUTPUT_STREAM,
    binary_predicate *bp) {
    return FALSE;
}
void Calculus::Equality::describe_for_index?Usage of Calculus::Equality::describe_for_index:
§2(bp_family *self, OUTPUT_STREAM,
    binary_predicate *bp) {
    WRITE("equality");
}
int Calculus::Equality::describe_empty_for_problems?Usage of Calculus::Equality::describe_empty_for_problems:
§2(bp_family *self, OUTPUT_STREAM,
    binary_predicate *bp) {
    return FALSE;
}
void Calculus::Equality::describe_empty_for_index?Usage of Calculus::Equality::describe_empty_for_index:
§2(bp_family *self, OUTPUT_STREAM,
    binary_predicate *bp) {
    WRITE("never-holding");
}