diff --git a/spot/twa/acc.hh b/spot/twa/acc.hh
index b552e907c..0b14c4112 100644
--- a/spot/twa/acc.hh
+++ b/spot/twa/acc.hh
@@ -686,7 +686,9 @@ namespace spot
 
       acc_code operator&(const acc_code& r)
       {
-        return *this &= r;
+        acc_code res = *this;
+        res &= r;
+        return res;
       }
 
       acc_code operator&(acc_code&& r)
@@ -789,9 +791,11 @@ namespace spot
         return res;
       }
 
-      acc_code& operator|(const acc_code& r)
+      acc_code operator|(const acc_code& r)
       {
-        return *this |= r;
+        acc_code res = *this;
+        res |= r;
+        return res;
       }
 
       acc_code& operator<<=(unsigned sets)
diff --git a/tests/python/acc_cond.ipynb b/tests/python/acc_cond.ipynb
index 759b8fa99..1922bcf23 100644
--- a/tests/python/acc_cond.ipynb
+++ b/tests/python/acc_cond.ipynb
@@ -15,7 +15,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.5.3+"
+   "version": "3.5.4"
   },
   "name": ""
  },
@@ -720,8 +720,8 @@
        "output_type": "stream",
        "stream": "stdout",
        "text": [
-        "((Fin(4) & Inf(5)) | (Fin(6) & Inf(7))) & ((Fin(0) & Inf(1)) | (Fin(2) & Inf(3)))\n",
-        "((Inf(4) | Fin(5)) & (Inf(6) | Fin(7))) | ((Inf(0) | Fin(1)) & (Inf(2) | Fin(3)))\n"
+        "(Fin(0) & Inf(1)) | (Fin(2) & Inf(3))\n",
+        "(Inf(0) | Fin(1)) & (Inf(2) | Fin(3))\n"
        ]
       }
      ],
@@ -1426,15 +1426,6 @@
       }
      ],
      "prompt_number": 52
-    },
-    {
-     "cell_type": "code",
-     "collapsed": false,
-     "input": [],
-     "language": "python",
-     "metadata": {},
-     "outputs": [],
-     "prompt_number": null
     }
    ],
    "metadata": {}