/**
 * Tiny assertion helper so every demo is self-checking: a claim that stops being true turns the
 * run red instead of quietly printing something different. Passing checks are echoed, so the
 * transcript shows exactly what was verified.
 */
final class Check {
    private Check() {}

    static void that(boolean condition, String claim) {
        if (!condition) {
            throw new AssertionError("CHECK FAILED: " + claim);
        }
        System.out.println("CHECK ok   : " + claim);
    }
}