Explanation

Why this crate exists

Engines are data-driven: a language definition supplies schemes and the engine interprets them (ADR-0044, «language as data»). That needs one shared term algebra — otherwise every engine invents its own node enum and kits can no longer swap engines. hnl-engine is that algebra, kept dependency-free so both the kernel (handl-language, handl-value) and every engine can sit above it without cycles.

Where it sits

This crate belongs to the engine-* family. Plugins, kits and hosts reach it through the kit's operation slot, so the engine can be swapped without touching a language definition.

See the reference for the API and the how-to guides for tasks.