Topic complements can be a subject, an adjective, a single word, or a group of words that behave like a subject or an adjective. Subject supplements can be subdivided into predicatnomenatives and predicates. The reason is very simple: whatever the subject, predictive syntax cooperates with the semantics of the singular noun to put the plural noun in a single collective bucket. An idiomatic use of the thing exerts the same collectivising influence on books: others have found that the grammatically erroneous wording is too strong. There is no “grammatical rule” for subject/complementary concordance. Please help me understand/improve my thinking here. It seems acceptable to establish a descriptor rule for students: the technical supplement normally corresponds to its subject. .