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

    multi method lb() { $!lb }
    multi method lb($lb) {
        die "Require lb <= ub" unless $lb <= $!ub;
        $!lb = $lb;
    }

    multi method ub() { $!ub }
    multi method ub($ub) {
        die "Require lb <= ub" unless $!lb <= $ub;
        $!ub = $ub;
    }
}