sub escape {
   my ($s) = @_;
   for ($s) {
      die "\"$1\" can't be produced by aXML safely"
         if /(&(?:lab|rab|lcb|rcb|lsb|rsb);)/;
      s/\</&lab;/g;
      s/\>/&rab;/g;
      s/\(/&lcb;/g;
      s/\)/&lcb;/g;
      s/\[/&lsb;/g;
      s/\]/&rsb;/g;
      return $_;
  }
}