Function trailingBecause

test/compute-engine/rule-aggregation.test.ts:108–111

(set: BoxedRuleSet, expr: BoxedExpression)

    return steps.length ? steps[steps.length - 1].value : expr;
  };
  const trailingBecause = (set: BoxedRuleSet, expr: BoxedExpression) => {
    const steps = replace(expr, set, REPLACE_OPTS);
    return steps.length ? steps[steps.length - 1].because : null;
  };

  it('multi-fire + head change: same final value as the linear scan', () => {
    // F(0) → F(1) → F(2) → G(99)