sub ave {
    my ($cnt , $tot);
    if ( @_ ) {
        $cnt += @_;
        $tot += $_ for @_;
        return $cnt ? $tot / $cnt : undef;
    }
    else {
        return sub {
            $cnt += @_;
            $tot += $_ for @_;
            return $cnt ? $tot / $cnt : undef;
        };
    }
}