class Interval {
    has Real $.lb = die 'Lower bound is required';
    has Real $.ub = die 'Upper bound is required';

    method lb() is rw {
        return Proxy.new:
            FETCH => method () { return $!lb; },
            STORE => method ($lb) {
                die "Require lb <= ub" unless $lb <= $!ub;
                $!lb = $lb;
            };
    }

    method ub() is rw {
        return Proxy.new:
            FETCH => method () { return $!ub; },
            STORE => method ($ub) {
                die "Require lb <= ub" unless $!lb <= $ub;
                $!ub = $ub;
            };
    }
}