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;