Indexed

The context-free specialization — the classical Grate shape (X = Unit), with the full-grid index. Generic andThen retains its product context even when both inputs have Unit context; no context-erasing coercion is installed in this prototype.

User-written to / from algebra. The caller is responsible for the laws stated on Indexed.

An Iso has one coordinate, not a broadcast over an arbitrary (possibly empty) index. The functions must be inverse in the monomorphic case, or have matching target-side inverses in the polymorphic case.

A cats Representable supplies precisely the two inverse tabulation operations needed here. Passed explicitly so its path-dependent Representation is retained without another given.

Identity algebra on a single Unit coordinate, including type-changing writes.