abstract sclass HAbstractRenderable {
  S baseLink;
  MakeFrame makeFrame;

  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) {
    if (makeFrame != null) ret makeFrame.makeFrame(title, contents);
    ret h1_title(title) + contents;
  }
}