book-pipeline held its own copy of the scheme operations bibclean already
had. The scheme itself — which headings a collection uses, their aliases,
their parents — stays with the collection, because that is the most
collection-specific thing there is. What a scheme *means* is the same
question everywhere, and this answers it: `canonical_subject`,
`compose_subjects`, `needs_subject`.
Checked against the Python over the library's 228 distinct tags and all
5,385 books: identical on every one.