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

### Example which does not work:
# class Interval {
#     has Real $.lb where * <= $.ub is rw = die 'Lower bound is required';
#     has Real $.ub where * >= $.lb is rw = die 'Upper bound is required';
#     after lb { say "Lower bound was changed!" if @_ }
# }

my $i = Interval.new( lb => 1, ub => 3);
say $i.perl;
$i.lb = 0;     # should print message
say $i.perl;
$i.lb = 6;     # should blow up!
say $i.perl;

##</code><code>##

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

    method update(:$lb = $.lb, :$ub = $.ub) {
        die "Require lb <= ub" unless $lb <= $ub;
        ($!lb, $!ub) = ($lb, $ub);
    }
}

##</code><code>##

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;
    }
}