abstract sclass HAbstractRenderable {
  S baseLink = "";
  MakeFrame makeFrame = (title, contents) -> h1_title(title) + contents;

  sinterface MakeFrame {
    S makeFrame(S title, S contents);
  }
  
  *() {}
  *(S *baseLink) {}
  
  HAbstractRenderable makeFrame(MakeFrame makeFrame) {
    this.makeFrame = makeFrame;
    this;
  }
  
  S baseLinkPlus(S uri) {
    ret nempty(uri) ? appendSlash(baseLink) + uri : baseLink;
  }
  
  // title is in HTML
  S frame(S title, S contents) {
    ret makeFrame.makeFrame(title, contents);
  }
  
  S refreshWithMsgs(S... msgs) { ret refreshWithMsgs(asList(msgs)); }
  
  S refreshWithMsgs(LS msgs, O... _) {
    optPar S anchor;
    optPar SS params;
    ret hrefresh(addAnchorToURL(appendQueryToURL(baseLink,
      mapPlus(params, msg := htmlEncode_nlToBr(lines_rtrim(msgs)))), anchor));
  }
  
  S renderMsgs(SS params) {
    ret pUnlessEmpty(params.get("msg"));
  }
}