sub AND {
    return _joiner( '&', @_ );
}

sub OR {
    return _joiner( '|', @_ );
}

sub _joiner {
    my $op = shift;
    my $filter = shift;
    while (my $cond = shift) {
        $filter = "($op $filter $cond)" if defined $cond;
    }
    return $filter;
}

sub NOT {
    return "(!$_[0])";
}

sub IGNORE {
    return;
}