A counter

has 'depth' => (
    metaclass => 'Counter',
    isa       => 'Int',
    is        => 'ro',
    default   => 0,
    provides  => {
       inc => 'ascend',
       dec => 'descend',
    },
);

before 'parse' => sub { $self->descend };
after  'parse' => sub { $self->ascend  };