sub new {
    my ($class, $str, @lists) = @_;
    my @list = map {'ARRAY' eq ref $_ ? @$_ : $_} @lists;
    return bless {str => $str, list => \@list}, $class;
}

##</code><code>##

sub catSF {
    my ($lhs, $rhs, $inv) = @_;

    ($lhs, $rhs) = ($rhs, $lhs) if $inv;