Special sentences for setting exotic plural forms of nouns.

§1. Sentences like "the plural of cherub is cherubim" are hardly needed now, because the inflections module now contains a full implementation of Conway's algorithm. Still, we keep the syntax around, and it may one day be useful again for languages other than English.

The subject phrase must match: 

<plural-sentence-subject> ::=
    <article> plural of <np-articled> |  ==> { pass 2 }
    plural of <np-articled>              ==> { pass 1 }

§2. Note that we are saved later grief by not allowing a plural form which would be illegal as a new noun: allowing "The plural of thing is ," would not end well.

Otherwise, though, we simply send the request to Pluralisation (in inflections). 

int Plurals::plural_SMF(int task, parse_node *V, wording *NPs) {
    wording SW = (NPs)?(NPs[0]):EMPTY_WORDING;
    wording OW = (NPs)?(NPs[1]):EMPTY_WORDING;
    switch (task) {  "The plural of seraph is seraphim."
        case ACCEPT_SMFT:
            if (<plural-sentence-subject>(SW)) {
                V->next = <<rp>>;
                <np-unparsed>(OW);
                V->next->next = <<rp>>;
                wording S = Node::get_text(V->next);
                wording P = Node::get_text(V->next->next);
                Forbid plural declarations containing quoted text2.1;
                if (Assertions::Creator::vet_name_for_noun(P) == FALSE) return TRUE;
                Pluralisation::register(S, P, DefaultLanguage::get(NULL));
                return TRUE;
            }
            break;
    }
    return FALSE;
}

§2.1. In general names of things which we need plurals for cannot contain quoted text anyway, so the following problem messages are not too gratuitous.

Forbid plural declarations containing quoted text2.1 = 

    LOOP_THROUGH_WORDING(i, S)
        if (Vocabulary::test_flags(i, TEXT_MC + TEXTWITHSUBS_MC)) {
            StandardProblems::sentence_problem(Task::syntax_tree(),
                _p_(PM_PluralOfQuoted),
                "declares a plural for a phrase containing quoted text",
                "which is forbidden. Sentences like this are supposed to "
                "declare plurals without quotation marks: for instance, "
                "'The plural of attorney general is attorneys general.'");
            return TRUE;
        }
    LOOP_THROUGH_WORDING(i, P)
        if (Vocabulary::test_flags(i, TEXT_MC + TEXTWITHSUBS_MC)) {
            StandardProblems::sentence_problem(Task::syntax_tree(),
                _p_(PM_PluralIsQuoted),
                "declares a plural for a phrase using quoted text",
                "which is forbidden. Sentences like this are supposed to "
                "declare plurals without quotation marks: for instance, "
                "'The plural of procurator fiscal is procurators fiscal.'");
            return TRUE;
        }