sub is_interesting {
    my $name = shift;

    # only directories are interesting
    return 0 unless -d $name;

    # and only those containing input.xml and output.xml
    return 0 unless -f $name . "/input.xml";
    return 0 unless -f $name . "/output.xml";

    return 1;
}

sub preproc {
    return grep { is_interesting($_) } @_;
}