main.c:711:5: error: expected expression
                                  move_t old_move;
                                  ^
  main.c:712:25: error: use of undeclared identifier 'old_move'
                                  if (undo_stack_pop(&old_move))
                                                      ^
  main.c:716:9: error: use of undeclared identifier 'old_move'
                                  x = old_move.x;
                                      ^
  main.c:717:9: error: use of undeclared identifier 'old_move'
                                  y = old_move.y;
                                      ^
  main.c:720:31: error: use of undeclared identifier 'old_move'
                                  user_board[posy*9+posx] = old_move.prev_val;
                                                            ^
  5 errors generated.