sclass StringBuilderWithIndent {
  new StringBuilder buf;
  S indent = "";
  
  void println(O o) {
    buf.append(indentx(indent, o + "\n"));
  }
  
  AutoCloseable indent() {
    S oldIndent = indent;
    indent += "  ";
    ret autoCloseable { indent = oldIndent; };
  }
  
  //void unindent() { indent = dropLast(2, indent); }
  
  toString { ret str(buf); }
}