// pred receives the text of opening tag (e.g. "<if $bla>")
sS html_evaluateIfTags(S html, IPred<S> pred) {
  LS tok = htmlTok(html);
  bool anyChange;
  while ping (true) {
    LS ifTag = first(findContainerTag(tok, "if"));
    if (ifTag == null) break;
    set anyChange;
    int iElse = indexOfIC(ifTag, "<else>");
    bool val = pred.get(second(ifTag));
    if (val) {
      // value is true, drop else branch (if there)
      if (iElse >= 0)
        clearTokens(ifTag, iElse, ifTag-2);
    } else
      // value is false, drop everything up until <else> (or whole thing)
      clearTokens(ifTag, 2, iElse < 0 ? l(ifTag) - 2 : iElse+1);
    }
      
    // clear opening and closing tag
    clearToken(ifTag, 1);
    clearToken(ifTag, l(ifTag)-2);
  }
  
  ret anyChange ? join(tok) : html;
}