sub odometer {
	die "no wheels specified" unless @_;
	my @w = map { [ $_, $_->() ] } @_;
	my $done;
	return sub {
		if ($done) { $done=0; return }
		my @cur = map { $_->[1] } @w;
		for ( my $i=$#w; $i>=0; $i-- ) {
			defined( $w[$i][1] = $w[$i][0]->() ) and last;
			$w[$i][1] = $w[$i][0]->();
			$done=1 unless $i;
		}
		return wantarray ? @cur : join '', map {defined()?$_:''} @cur;
	};
}