sclass LastLineBuffer {
  int lineCount;
  S lastLine, currentLine = "";
  
  void append(S s) {
    currentLine += s;
    int i;
    while ((i = currentLine.indexOf('\n')) >= 0) {
      // inefficient
      lastLine = substring(currentLine, 0, i);
      currentLine = substring(currentLine, i+1);
    }
  }
  
  S lastCompleteLine() { ret lastLine; }
}