fn rule_b_to_c(_: &mut Commands<Expr, ()>, expr: &Expr, _: &()) -> Option<Expr> {

    if let Expr::B = expr {

        return Some(Expr::C);

    }

    None

}

fn closure_rules() {

    let expr = Expr::A;

    let engine = EngineBuilder::new()

        .add_rule(

            (|_, t, _| match t {

                Expr::A => Some(Expr::B),

                _ => None,

            }) as RuleFn<_, _>,

        .add_rule_group(rule_fns![

            |_, t, _| match t {

                Expr::C => Some(Expr::D),

                _ => None,

            },

        .build();

    let (result, _) = engine.morph(expr, ());

    assert_eq!(result, Expr::D);

}