diff --git a/spot/tl/formula.hh b/spot/tl/formula.hh
index 6dad5a318..f104bf496 100644
--- a/spot/tl/formula.hh
+++ b/spot/tl/formula.hh
@@ -799,6 +799,20 @@ namespace spot
       return formula(fnode::ap(name));
     }
 
+    /// \brief Build an atomic proposition from... an atomic proposition.
+    ///
+    /// The only practical interest of this methods is for the Python
+    /// bindings, where ap() can therefore work both from string or
+    /// atomic propositions.
+    static formula ap(const formula& a)
+    {
+      if (a.kind() == op::ap)
+        return a;
+      else
+        throw std::invalid_argument("atomic propositions cannot be "
+                                    "constructed from arbitrary formulas");
+    }
+
     /// \brief Build a unary operator.
     /// \pre \a o should be one of op::Not, op::X, op::F, op::G,
     /// op::Closure, op::NegClosure, op::NegClosureMarked.
diff --git a/tests/python/ltsmin.ipynb b/tests/python/ltsmin.ipynb
index 2401b5f70..069a0c5e1 100644
--- a/tests/python/ltsmin.ipynb
+++ b/tests/python/ltsmin.ipynb
@@ -18,7 +18,7 @@
    "version": "3.5.1"
   },
   "name": "",
-  "signature": "sha256:0c10865a9a58374d983e19129ce565c14e590067c1eedf4f12dfe62fa3f58a02"
+  "signature": "sha256:d1444ee3c1e56aaa453ac171d129f0ba4486e1e194debc51965d45429a844334"
  },
  "nbformat": 3,
  "nbformat_minor": 0,
@@ -447,7 +447,7 @@
         "</svg>\n"
        ],
        "text": [
-        "<spot.impl.kripke; proxy of <Swig Object of type 'std::shared_ptr< spot::kripke > *' at 0x7fda50a73c90> >"
+        "<spot.impl.kripke; proxy of <Swig Object of type 'std::shared_ptr< spot::kripke > *' at 0x7f7b280d08a0> >"
        ]
       }
      ],
@@ -726,7 +726,7 @@
         "</svg>"
        ],
        "text": [
-        "<IPython.core.display.SVG at 0x7fda50a09710>"
+        "<IPython.core.display.SVG at 0x7f7b280dd630>"
        ]
       }
      ],
@@ -1082,7 +1082,7 @@
         "</svg>"
        ],
        "text": [
-        "<IPython.core.display.SVG at 0x7fda58a31940>"
+        "<IPython.core.display.SVG at 0x7f7b110b9c50>"
        ]
       }
      ],
@@ -1152,7 +1152,7 @@
         "</svg>\n"
        ],
        "text": [
-        "<spot.impl.twa_graph; proxy of <Swig Object of type 'std::shared_ptr< spot::twa_graph > *' at 0x7fda58aa2270> >"
+        "<spot.impl.twa_graph; proxy of <Swig Object of type 'std::shared_ptr< spot::twa_graph > *' at 0x7f7b2fcf8090> >"
        ]
       }
      ],
@@ -1314,11 +1314,46 @@
         "</svg>\n"
        ],
        "text": [
-        "<spot.impl.twa_product; proxy of <Swig Object of type 'std::shared_ptr< spot::twa_product > *' at 0x7fda50a736f0> >"
+        "<spot.impl.twa_product; proxy of <Swig Object of type 'std::shared_ptr< spot::twa_product > *' at 0x7f7b280d08d0> >"
        ]
       }
      ],
      "prompt_number": 13
+    },
+    {
+     "cell_type": "code",
+     "collapsed": false,
+     "input": [
+      "def model_check(f, m):\n",
+      "    f = spot.formula(f)\n",
+      "    ss = m.kripke(spot.atomic_prop_collect(f))\n",
+      "    nf = spot.formula_Not(f).translate()\n",
+      "    return spot.otf_product(ss, nf).is_empty() "
+     ],
+     "language": "python",
+     "metadata": {},
+     "outputs": [],
+     "prompt_number": 14
+    },
+    {
+     "cell_type": "code",
+     "collapsed": false,
+     "input": [
+      "model_check('\"a<1\" R \"b > 1\"', m)"
+     ],
+     "language": "python",
+     "metadata": {},
+     "outputs": [
+      {
+       "metadata": {},
+       "output_type": "pyout",
+       "prompt_number": 15,
+       "text": [
+        "False"
+       ]
+      }
+     ],
+     "prompt_number": 15
     }
    ],
    "metadata": {}