diff --git a/src/statebag.py b/src/statebag.py
--- a/src/statebag.py
+++ b/src/statebag.py
@@ -16,19 +16,29 @@ So we try to find the correct crossover 
 	- remember the better variant
 - linearize the fork back by discarding s'_j-s preceding the crossover and s_j-s following the crossover
 """
+import random
 
 from util import EMPTY
-from go.core import transitionSequence
+from go.engine import transitionSequence
+
+rand=random.Random()
+rand.seed(361)
+zobristNums=tuple(
+	tuple(
+		tuple(rand.getrandbits(32) for i in range(3)) for c in range(19)
+	) for r in range(19)
+)
 
 
 ## Crude lower bound on edit distance between states.
 def estimateDistance(diff):
+	# lot of room for improvements
 	additions=sum(1 for d in diff if d[2]=="+")
 	deletions=sum(1 for d in diff if d[2]=="-")
 	replacements=len(diff)-additions-deletions
 	if additions>0: return additions+replacements
 	elif replacements==0 and deletions>0: return 2 # take n, return 1
-	return replacements # ???
+	return replacements+1 # ???
 
 
 class BoardState:
@@ -40,6 +50,13 @@ class BoardState:
 		self.weight=0
 		self.diff2Prev=None
 
+	def hash(self):
+		res=0
+		for (r,row) in enumerate(self._board):
+			for (c,item) in enumerate(row):
+				res^=zobristNums[r][c][item+1]
+		return res
+
 	def __iter__(self): return iter(self._board)
 
 	def __getitem__(self,key): return self._board[key]
@@ -52,9 +69,13 @@ class BoardState:
 				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,item2,item1)) # from->to
+				else: res.append((r,c,"*",item1)) # ->to
 		return res
 
+	def __eq__(self,x):
+		# might want to use hash
+		return self._board==x._board
+
 
 class StateBag:
 	def __init__(self):
@@ -62,6 +83,8 @@ class StateBag:
 
 	def pushState(self,board):
 		sn=BoardState(board)
+		if self._states and sn==self._states[-1]: return # no change
+
 		for s in reversed(self._states):
 			diff=sn-s
 			distEst=estimateDistance(diff)