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";
    }
    eval ("sub $AUTOLOAD {
        my (\$self,\$value) = \@_;
        \$self->{$name} = \$value if defined \$value;
        \$self->{$name};
    }");
    die $@ if $@;
    goto &$AUTOLOAD;
}