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";
        }

        if (@_) {
            return $self->{$name} = shift;
        } else {
            return $self->{$name};
        }
    }