sub new {
    my $class = shift;

    my $self = {
        FLYBEHAVIOUR   => undef,
        QUACKBEHAVIOUR => undef,
    };
    my @protected = qw/FLYBEHAVIOUR QUACKBEHAVIOUR/;
    my @private= qw//;

    my $closure = sub {
        my $field = shift;
        grep( /$field$/, @private ) and caller(0) eq __PACKAGE__ 
            || confess "$field is private";
        grep( /$field/, @protected ) and caller(0)->isa(__PACKAGE__)
            || confess "$field is protected";
        if (@_) { $self->{$field} = shift; }
        return $self->{$field};
    };

    bless( $closure, $class );
    return $closure;
}