refractoring

app/src/main/java/sudoku/constraint/BlockConstraint.java

@@ -2,6 +2,7 @@ package sudoku.constraint;
 
 import sudoku.Block;
 import sudoku.Cell;
+import sudoku.MutableCell;
 import sudoku.Sudoku;
 
 public class BlockConstraint implements IConstraint{
@@ -9,7 +10,7 @@ public class BlockConstraint implements IConstraint{
 	@Override
 	public boolean canBePlaced(final Sudoku s, int x, int y, int newSymbolIndex) {
 		Block bloc = s.getCell(x, y).getBlock();
-		return !bloc.getCells().contains(new Cell(newSymbolIndex));
+		return !bloc.getCells().contains(new MutableCell(newSymbolIndex));
 	}
 
 }