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<b>Warning</b>:  session_start(): Failed to read session data: files (path: /var/lib/php/sessions) in <b>/var/www/tb-usercake/models/config.php</b> on line <b>51</b><br />
sclass BStack > VStack {
  // alternative to backtrack to
  IF0<BStack> alternative;
  
  // steps to undo before switching to the alternative
  L<AutoCloseable> undos;
  
  BStack cloneStack() {
    new BStack s;
    s.stack = shallowCloneElements(stack);
  }
  
  void addUndo(AutoCloseable undo) {
    if (undo == null) ret;
    if (undos == null) undos = new L;
    undos.add(undo);
  }
  
  BStack addAlternative(IVF1<BStack> action) {
    BStack clonedStack = cloneStack();
    alternative = -> {
      action.get(clonedStack);
      ret clonedStack;
    };
    ret clonedStack;
  }
  
  BStack backtrack() {
    if (undos != null)
      for (undo : reversed(getAndClear(undos)))
        undo.close();
    ret alternative?!;
  }
}