diff --git a/src/go/core.py b/src/go/core.py
--- a/src/go/core.py
+++ b/src/go/core.py
@@ -5,12 +5,20 @@ from .helperboard import HelperBoard
 from .gamerecord import GameRecord
 
 
+PASS=(99,99)
+
+
 class Go:
 	## Initializes self.board to a list[r][c]=EMPTY.
 	def __init__(self,boardSize=19):
 		self.boardSize=boardSize
 		self.board=[[EMPTY]*boardSize for x in range(boardSize)]
 		self.toMove=BLACK
+
+		# utility field to allow undoing moves. but only useful right after doMove. this class doesn't need it, so it is not kept correct at other times
+		self.captures=[None]*4
+		self.captureCount=0
+
 		self._helper=HelperBoard(self.board)
 		self._freshHash=hashBoard(self.board) # always reflecting current state of the board
 		self._hashes=[self._freshHash]
@@ -24,15 +32,23 @@ class Go:
 	#  @return True on success, False on failure (illegal move)
 	def doMove(self,color,r,c):
 		if color!=self.toMove: log.warning("move by %s out of order",colorNames[color])
+		if (r,c)==PASS:
+			self.toMove*=-1 # pass
+			self._hashes.append(self._freshHash)
+			return True
 		if self.board[r][c]!=EMPTY: return False
 
 		self.board[r][c]=color
 		self._freshHash^=diffHash(r,c,EMPTY,color)
 
 		# capture neighbors
+		self.captureCount=0
 		for dr,dc in ((-1,0),(1,0),(0,-1),(0,1)):
 			self._helper.clear()
-			if not self._helper.floodFill(-color,r+dr,c+dc,EMPTY): self._remove()
+			if not self._helper.floodFill(-color,r+dr,c+dc,EMPTY):
+				self.captures[self.captureCount]=(r+dr,c+dc)
+				self.captureCount+=1
+				self._remove()
 
 		# check for suicide and prevent it
 		self._helper.clear()
@@ -46,15 +62,16 @@ class Go:
 		return True
 
 	def undoMove(self,r,c,captures):
-		assert self.board[r][c]==-1*self.toMove, "{0}!={1}".format(self.board[r][c],-1*self.toMove)
+		if (r,c)!=PASS:
+			assert self.board[r][c]==-1*self.toMove, "{0}!={1}".format(self.board[r][c],-1*self.toMove)
 
-		if len(captures)>0:
-			self._helper.clear()
-			for (ri,ci) in captures:
-				self._helper.floodFill(EMPTY,ri,ci)
-			self._fill(self.toMove)
+			if len(captures)>0:
+				self._helper.clear()
+				for (ri,ci) in captures:
+					self._helper.floodFill(EMPTY,ri,ci)
+				self._fill(self.toMove)
 
-		self.board[r][c]=EMPTY
+			self.board[r][c]=EMPTY
 		self.toMove*=-1
 		self._hashes.pop()
 		self._freshHash=self._hashes[-1]