An archetype is data.

Instrument taxonomies are where class hierarchies go to grow teeth: equity, future, option, each sharing margin and settlement machinery, each diverging somewhere — and twenty years of inherited game-engine wisdom says the subclass tree ends in tears. West's component turn was the industry-scale retreat from exactly this shape, and the Gang of Four had stated the principle a decade earlier: favor composition over class inheritance, a judgment Gamma stood by — “I still think it's true even after ten years” — because the base-subclass coupling is implicit and brittle. BCS gives the principle its data-only form, so the trading desk's “all index futures now mark daily” is an edit to one entity, not a redeploy of a class.

The full committed transcript, verbatim — boot, then A1 through A5:

boot: the registry grows by one -- ARC, archetype definitions as rows
A1 order ok -- composition order proven: tick 0.25 from the instrument, settlement :daily_mark from the archetype, margin true from the base
A2 amplify ok -- one definition edited, a thousand instruments follow: multiplier 100 at next read; an instrument row is 18 words, its composed view 14
A3 guard ok -- the diamond is unrepresentable (one :extends) and the cycle is refused: {:error, :cycle}; depth capped at 8: {:error, :depth}
A4 rows ok -- archetypes are rows: an AST name refused with {:error, :namespace}; ARC pages newest-first like any property
A5 cost ok -- read-time composition over a depth-2 chain: store-lane 5442 ns/op vs snapshot-lane 1048 ns/op; a bare row get is 2653 ns/op
PASS 5/5

The resolver lives at runtimes/elixir/lib/echo_data/bcs/archetypes.ex; it is pure by construction — it takes a fetch function, so the same walk runs against a store boundary or a snapshot, and the fetch-function seam doubles as the test seam. The third lane delivers the chapter's best sentence: the snapshot-lane resolve — the full composition, pure, over a defs snapshot — is cheaper than a single bare get through the boundary. Composition was never the cost; the boundary crossing was. In the Part II arc the module stands between The CHAMP Property Database (B2.3) and Relations Are Systems (B2.5).