What changed:
- budget_left(Budget, Spent, Left) = plus(Spent, Left, Budget) — a budget where any one of the three values can be the unknown (left / spent / budget).
- next_page(Page, Next) = succ(Page, Next) — next page forward, previous page backward.
- Prose now opens by showing the three separate is formulas budget_left would otherwise need, then collapses them into one plus — so the "why not just use is?" doubt gets answered on the page instead of left hanging. The lesson lands as: a relation isn't a weaker is; it's is in every direction at once.

Registry name stayed 05-both-ways, so no info.toml change. The test
exercises both predicates in every direction (the thing an is-based
version couldn't pass).

That addresses the wrapper-makes-it-look-useless problem you flagged —
the builtins now have a concrete job and a visible payoff. Chapter 14
still awaits your explicit go-ahead.

navicore merged commit c30a4558e3 into main

2026-06-24 18:29:27 +00:00

navicore deleted branch chapter13

2026-06-24 18:29:27 +00:00

navicore referenced this pull request from a commit

2026-06-24 18:29:27 +00:00

Merge pull request '13' (#14) from chapter13 into main