sub readbackwards_utf8 { # returns an iterator
	my ($fn, $window) = @_;
	die "Bad window $window" unless $window>=4;
	open my $fh, '<:raw', $fn or die "open $fn: $!";
	my $curpos = -s $fh;
	return sub {
		if ( $curpos<1 ) { close $fh if $fh; $fh=undef; return }
		my $bytes = $curpos < $window ? $curpos : $window;
		seek($fh, $curpos-=$bytes, 0) or die "seek $curpos $fn: $!";
		read($fh, my $buf, $bytes) == $bytes
			or die "read $bytes bytes at $curpos from $fn: $!";
		while ( (ord(substr $buf, 0, 1) & 0b11000000)==0b10000000 )
			{ $buf = substr $buf, 1; $curpos++ }
		utf8::decode($buf);
		return $buf;
	}
}