diff --git a/Exercise sheet 4/exercise_sheet_04.ipynb b/Exercise sheet 4/exercise_sheet_04.ipynb
index 326aab0..ee957cc 100644
--- a/Exercise sheet 4/exercise_sheet_04.ipynb	
+++ b/Exercise sheet 4/exercise_sheet_04.ipynb	
@@ -916,7 +916,8 @@
     "def remains_valid_after_move(x,i,next_position,L):\n",
     "    '''Returns True if replacing x[i] by next_position would yield a valid configuration,\n",
     "    otherwise False.'''\n",
-    "    # TODO: Do I need to operate on a copy?\n",
+    "    # We need to create a copy as not to alter the original x\n",
+    "    # outside this function.\n",
     "    copy_x = np.copy(x)\n",
     "    copy_x[i] = next_position\n",
     "    return disk_config_valid(copy_x, L)"