A.6.B:8.2 - Show #1: Effect signature vs handler (post‑2015 effect systems)

A service boundary naturally mirrors algebraic effects & handlers practice (popularized broadly in the post‑2015 era, with mainstream effect handlers becoming especially prominent around OCaml 5):

• L: defines the operation vocabulary and laws (effect signature semantics).
• A: defines when the operation is admissible (runtime guard predicates).
• D: states who must enforce guards and what the provider commits to (operator and implementer duties; SLAs).
• E: ties “what happened” to observable carriers (traces, logs, metrics, and events) so commitments can be adjudicated.

The square prevents accidentally writing handler obligations as laws or treating observability as a definition.