fix setCell

app/src/main/java/sudoku/Sudoku.java

@@ -31,7 +31,12 @@ public class Sudoku {
 				return false;
 			}
 		}
-		return true;
+		Cell cell = getCell(x, y);
+		if (cell instanceof MutableCell mCell) {
+			mCell.setSymbolIndex(value);
+			return true;
+		}
+		return false;
 	}
 
 	public Cell getCell(int x, int y) {