sub AUTOLOAD {
    my $self = shift;
    my $type = ref($self)
        or croak "$self is not an object";

    my $name = $AUTOLOAD;
    $name =~ s/.*://;   # strip fully-qualified portion

    unless (exists $self->{_permitted}->{$name} ) {
        croak "Can't access `$name' field in class $type";
    }
    my $sub = sub {
        my ($self,$value) = @_;
        $self->{$name} = $value if defined $value;
        $self->{$name};
    };
    {
        no strict 'refs';
        *{$name} = $sub;
    }
    goto $sub;
}