diff --git a/wrap/python/spot.py b/wrap/python/spot.py
index bd2d9049c..668cc9878 100644
--- a/wrap/python/spot.py
+++ b/wrap/python/spot.py
@@ -417,11 +417,10 @@ def _addfilter(fun):
     def nfiltf(self, *args, **kwargs):
         it = filter(lambda f: not getattr(f, fun)(*args, **kwargs), self)
         return formulaiterator(it)
-    setattr(formulaiterator, fun, filtf)
     if fun[:3] == 'is_':
-        notfun = fun[:3] + 'not_' + fun[3:]
+        notfun = 'is_not_' + fun[3:]
     elif fun[:4] == 'has_':
-        notfun = fun[:4] + 'no_' + fun[4:]
+        notfun = 'has_no_' + fun[4:]
     else:
         notfun = 'not_' + fun
     setattr(formulaiterator, fun, filtf)
@@ -433,9 +432,9 @@ def _addfilter(fun):
 def _addmap(fun):
     def mapf(self, *args, **kwargs):
         return formulaiterator(map(lambda f: getattr(f, fun)(*args, **kwargs),
-self))
-    setattr(formula, fun, lambda self, *args, **kwargs: globals()[fun](self,
-    *args, **kwargs))
+                                   self))
+    setattr(formula, fun,
+            lambda self, *args, **kwargs: globals()[fun](self, *args, **kwargs))
     setattr(formulaiterator, fun, mapf)
 
 def randltl(ap, n = -1, **kwargs):
@@ -531,18 +530,26 @@ def randltl(ap, n = -1, **kwargs):
             sys.stderr.write("internal error: unknown type of output")
         return
 
-    def _randltlgenerator(rg):
-        i = 0
-        while i != n:
-            f = rg.next()
+    class _randltliterator:
+        def __init__(self, rg, n):
+            self.rg = rg
+            self.i = 0
+            self.n = n
+        def __iter__(self):
+            return self
+        def __next__(self):
+            if self.i == self.n:
+                raise StopIteration
+            f = self.rg.next()
             if f is None:
-                sys.stderr.write("Warning: could not generate a new unique formula " \
-                "after " + str(MAX_TRIALS) + " trials.\n")
-                yield None
-            else:
-                yield f
-                i += 1
-    return formulaiterator(_randltlgenerator(rg))
+                sys.stderr.write("Warning: could not generate a new "
+                                 "unique formula after {} trials.\n"
+                                 .format(MAX_TRIALS))
+                raise StopIteration
+            self.i += 1
+            return f
+
+    return formulaiterator(_randltliterator(rg, n))
 
 def simplify(f, **kwargs):
     level = kwargs.get('level', None)
diff --git a/wrap/python/tests/randltl.ipynb b/wrap/python/tests/randltl.ipynb
index cdd2a3acd..fd163cbce 100644
--- a/wrap/python/tests/randltl.ipynb
+++ b/wrap/python/tests/randltl.ipynb
@@ -1,7 +1,23 @@
 {
  "metadata": {
-  "name": "",
-  "signature": "sha256:54b0c9570ca91322c466f914d622a3723cd3e450612a055a188927bf50dfdaf8"
+  "kernelspec": {
+   "display_name": "Python 3",
+   "language": "python",
+   "name": "python3"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 3
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython3",
+   "version": "3.4.3+"
+  },
+  "name": ""
  },
  "nbformat": 3,
  "nbformat_minor": 0,
@@ -164,7 +180,7 @@
        "output_type": "stream",
        "stream": "stdout",
        "text": [
-        "G(p2 U Gp0)\n"
+        "G(p1 U Gp0)\n"
        ]
       }
      ],
@@ -199,7 +215,7 @@
        "output_type": "stream",
        "stream": "stdout",
        "text": [
-        "{{p0 && p1}[*]}<>-> (Fp1 & Fp0)\n"
+        "{{p0 && p2}[*]}<>-> (Fp2 & Fp0)\n"
        ]
       }
      ],
@@ -571,11 +587,11 @@
        "stream": "stdout",
        "text": [
         "0\n",
-        "!(F!p0 M 1)\n",
-        "(Gp1 | Fp0) M 1\n",
-        "F!(!p0 <-> FGp0)\n",
-        "Gp0 U (p0 U GFp0)\n",
-        "(!p0 U p0) U ((p1 & (p1 U (!p1 & (!p1 -> Fp0))) & ((!p0 U !p1) | (p0 U !p1))) | (!p1 & (!p1 U (p1 & (!p1 -> Fp0))) & ((!p0 U p1) | (p0 U p1))) | (p0 & (p0 U (!p0 & (!p1 -> Fp0))) & ((!p1 U !p0) | (p1 U !p0))) | (!p0 & (!p0 U (p0 & (!p1 -> Fp0))) & ((!p1 U p0) | (p1 U p0))) | ((!p1 -> Fp0) & (Gp1 | G!p1) & (Gp0 | G!p0)))\n"
+        "!(F!p1 M 1)\n",
+        "(Gp0 | Fp1) M 1\n",
+        "F!(!p1 <-> FGp1)\n",
+        "Gp1 U (p1 U GFp1)\n",
+        "(!p1 U p1) U ((p0 & (p0 U (!p0 & (!p0 -> Fp1))) & ((!p1 U !p0) | (p1 U !p0))) | (!p0 & (!p0 U (p0 & (!p0 -> Fp1))) & ((!p1 U p0) | (p1 U p0))) | (p1 & (p1 U (!p1 & (!p0 -> Fp1))) & ((!p0 U !p1) | (p0 U !p1))) | (!p1 & (!p1 U (p1 & (!p0 -> Fp1))) & ((!p0 U p1) | (p0 U p1))) | ((!p0 -> Fp1) & (Gp0 | G!p0) & (Gp1 | G!p1)))\n"
        ]
       }
      ],