diff --git a/src/go/core.py b/src/go/core.py
--- a/src/go/core.py
+++ b/src/go/core.py
@@ -1,6 +1,6 @@
 import logging as log
 
-from util import EMPTY,BLACK,WHITE,colorNames
+from util import EMPTY,BLACK,WHITE, colorNames,hashBoard
 from .helperboard import HelperBoard
 from .gamerecord import GameRecord
 
@@ -12,6 +12,7 @@ class Go:
 		self.board=[[EMPTY]*boardSize for x in range(boardSize)]
 		self.toMove=BLACK
 		self._helper=HelperBoard(self.board)
+		self._hashes=[]
 		self._record=GameRecord()
 
 	def listMoves(self,diff=[]):
@@ -41,15 +42,19 @@ class Go:
 			return False
 		self._record.move(color,row,col)
 		self.toMove=-1*color
+		self._hashes.append(self.hash())
 		return True
 
-	def undoMove(self,color,r,c,captures):
-		self.board[r][c]=color
+	def undoMove(self,r,c,captures):
+		assert self.board[r][c]==-1*self.toMove
+		self.toMove*=-1
+		self.board[r][c]=self.toMove
 		if len(captures)>0:
 			self._helper.clear()
 			for (r,c) in captures:
 				self._helper.floodFill(EMPTY,r,c)
-			self._fill(-color)
+			self._fill(-self.toMove)
+		self._hashes.pop()
 
 	def transitionMove(self,board):
 		res=transitionMove(self.board,board)
@@ -57,6 +62,9 @@ class Go:
 		(r,c,color)=res
 		return self.doMove(color,r,c)
 
+	def hash(self):
+		return hashBoard(self.board)
+
 	## Removes stones at coordinates marked with True in self.helper.
 	def _remove(self):
 		self._fill(EMPTY)