anchor.diff:        if (notOnGround(move)){
           //pruefen ob der Tetromino mit einem bereits gesetzten Tetromino kollidiert
           if(!tetrominoCollision(move)) {
  -          //den Tetromino vin der alten Position entfernen
  +          //den Tetromino in der alten Position entfernen
             this._stone.forEach((piece) {
               this._game._field[piece['row']][piece['col']].isActive = false;
               this._game._field[piece['row']][piece['col']].color = #empty;
  
positive.code:           this._stone.forEach((piece) {
              this._game._field[piece['row']][piece['col']].isActive = true;
            });
          } else{
            //den Tetromino bei einer Kollision an der Position setzen
            this._stone.forEach((piece) {
              this._game._field[piece['row']][piece['col']].isActive = false;
            });
            //TODO: check if a row was completed
            nextTetromino();
          }
        } else {
          this._stone.forEach((piece) {
  
negative.code:       //    'COLOR: ${this._game.field[cell['row']][cell['col']].color}');
        if(this._game.field[cell['row']][cell['col']].color != #empty &&
            !this._game.field[cell['row']][cell['col']].isActive){
            return true;
        }
      }
  
  