
Undeployable placeholder for sketching backend intent before a real component exists. Variadic [$a] inputs and [$b] outputs use unrelated typevars; a readme config key holds the intent. Strict inference refuses any backend with it.
Stand in for a missing scorer between a real source and a real sink while the graph plumbing is validated; the intended contract lives in the readme note.
Hand-off marker between two halves of a backend when the connector has not been built yet.
[$a] input and [$b] output expansions are distinct typevars, so the placeholder produces no concrete output types and any consumer wired to its outputs is left with unresolved inputs.