sub AUTOLOAD {
    my ($self, $newval) = @_;
    $AUTOLOAD =~ s/.*:://;
    my $attr;
    if (exists $self->{$attr=$AUTOLOAD}) {
        # public attribute always accessible
    }
    elsif (exists $self->{$attr="_$AUTOLOAD"}) {
        croak "Cannot access protected attribute '$attr'"
            unless caller().isa(ref $self);
    }
    elsif (exists $self->{$attr="__$AUTOLOAD"}) {
        croak "Cannot access private attribute '$attr'"
            unless caller() eq ref $self;
    }
    else {
        croak "No such attribute ($AUTOLOAD) for object"
    }
    $self->{$attr} = $newval if @_ > 1;
    return $self->{$attr};
}