function guardarBoard() {
  board_copy = [];
  for (var i = 0; i < board.length; i++) {
      row = board[i];
      for (var j = 0; j < row.length; j++) {
          board_copy[i][j] = row[j];
      }
  }
  board_history.push(board_copy);
}

function undo() {
  board = board_history.pop();
}