diff --git a/src/statebag/boardstate.py b/src/statebag/boardstate.py
--- a/src/statebag/boardstate.py
+++ b/src/statebag/boardstate.py
@@ -63,15 +63,16 @@ class BoardState:
 
 	def __getitem__(self,key): return self._board[key]
 
-	def __sub__(self,x):
+	## Compute difference self-s.
+	def __sub__(self,s):
 		res=[]
 
-		for (r,(row1,row2)) in enumerate(zip(self._board,x)):
-			for (c,(item1,item2)) in enumerate(zip(row1,row2)):
-				if item1==item2: continue
-				elif item2==EMPTY: res.append((r,c,"+",item1))
-				elif item1==EMPTY: res.append((r,c,"-",item2))
-				else: res.append((r,c,"*",item1)) # ->to
+		for (r,(row,rowS)) in enumerate(zip(self._board,s)):
+			for (c,(item,itemS)) in enumerate(zip(row,rowS)):
+				if item==itemS: continue
+				elif itemS==EMPTY: res.append((r,c,"+",item))
+				elif item==EMPTY: res.append((r,c,"-",itemS))
+				else: res.append((r,c,"*",item)) # ->to
 		return res
 
 	def __eq__(self,x):