diff --git a/lib/goban.py b/lib/goban.py
index 8edfc86..d4625bb 100644
--- a/lib/goban.py
+++ b/lib/goban.py
@@ -50,8 +50,6 @@ class Goban:
         if not self._valid_move(rpos):
             return
 
-        # fixme - update the GRAY status of affected squares
-
         self.board[rpos] = self.to_move
         self._capture(rpos)
 
@@ -81,8 +79,8 @@ class Goban:
             return False
 
         # Can't play atop another stone
-        if self.board[pos] == Goban.EMPTY:
-            return True
+        if self.board[pos] != Goban.EMPTY:
+            return False
 
         # Temporarily place the stone
         self.board[pos] = self.to_move
@@ -113,7 +111,7 @@ class Goban:
             return 0
         
         if checked is None:
-            bs = board_size * board_size
+            bs = self.board_size * self.board_size
             checked = [False] * bs
 
         if checked[pos]:
@@ -156,7 +154,6 @@ class Goban:
         self.board[x][y].state = Goban.EMPTY
 
 
-    # fixme - needs to be evaluated with new data model
     def _delete_group_r(self, pos, who):
         if not self._on_board(pos):
             return
@@ -187,9 +184,9 @@ class Goban:
             if point == Goban.EMPTY:
                 s = img_res[self.def_draw_codes[pos]]
             elif point == Goban.BLACK:
-                s = img_res('b')
+                s = img_res['b']
             elif point == Goban.WHITE:
-                s = img_res('w')
+                s = img_res['w']
 
             s = pygame.transform.scale(s, (inc, inc))
             ret.blit(s, ((pos % self.board_size) *inc, (pos / self.board_size) *inc))
@@ -263,7 +260,7 @@ class Goban:
 
 
     def _on_board(self, pos):
-        return pos < 0 or pos > self.board_size * self.board_size - 1
+        return pos >= 0 or pos < self.board_size * self.board_size
 
 
     def _other_color(self, color):