sub of {
    my $x = shift;

    my @y = map { $_-1 if defined && /^\d+$/ && $_>0 } @_;
    # protect the caller from placement errors
    return undef unless ($#y == $#_);

    return map { ($_<$$x{'n'}) ? $$x{'perm'}[$_] + 1 : $_+1 } @y;
}