diff --git a/stl/ast.py b/stl/ast.py
index 0d1c40f..e16a934 100644
--- a/stl/ast.py
+++ b/stl/ast.py
@@ -41,7 +41,7 @@ class AST(object):
 
     @property
     def children(self):
-        return set()
+        return tuple()
 
     def walk(self):
         """Walk of the AST."""
@@ -123,7 +123,7 @@ class AtomicPred(namedtuple("AP", ["id"]), AST):
 
     @property
     def children(self):
-        return set()
+        return tuple()
 
 
 class LinEq(namedtuple("LinEquality", ["terms", "op", "const"]), AST):
@@ -134,7 +134,7 @@ class LinEq(namedtuple("LinEquality", ["terms", "op", "const"]), AST):
 
     @property
     def children(self):
-        return set()
+        return tuple()
 
     def __hash__(self):
         # TODO: compute hash based on contents
@@ -171,7 +171,7 @@ class NaryOpSTL(namedtuple('NaryOp', ['args']), AST):
 
     @property
     def children(self):
-        return set(self.args)
+        return tuple(self.args)
 
 
 class Or(NaryOpSTL):
@@ -203,7 +203,7 @@ class ModalOp(namedtuple('ModalOp', ['interval', 'arg']), AST):
 
     @property
     def children(self):
-        return {self.arg}
+        return (self.arg,)
 
 
 class F(ModalOp):
@@ -232,7 +232,7 @@ class Until(namedtuple('ModalOp', ['arg1', 'arg2']), AST):
 
     @property
     def children(self):
-        return {self.arg1, self.arg2}
+        return (self.arg1, self.arg2)
 
     def __hash__(self):
         # TODO: compute hash based on contents
@@ -247,7 +247,7 @@ class Neg(namedtuple('Neg', ['arg']), AST):
 
     @property
     def children(self):
-        return {self.arg}
+        return (self.arg,)
 
     def __hash__(self):
         # TODO: compute hash based on contents
@@ -262,7 +262,7 @@ class Next(namedtuple('Next', ['arg']), AST):
 
     @property
     def children(self):
-        return {self.arg}
+        return (self.arg,)
 
     def __hash__(self):
         # TODO: compute hash based on contents
diff --git a/stl/test_utils.py b/stl/test_utils.py
new file mode 100644
index 0000000..250ad54
--- /dev/null
+++ b/stl/test_utils.py
@@ -0,0 +1,13 @@
+import stl
+from stl.hypothesis import SignalTemporalLogicStrategy
+
+from hypothesis import given
+
+
+@given(SignalTemporalLogicStrategy)
+def test_f_neg_or_canonical_form(phi):
+    phi2 = stl.utils.f_neg_or_canonical_form(phi)
+    phi3 = stl.utils.f_neg_or_canonical_form(phi2)
+    assert phi2 == phi3
+    assert not any(
+        isinstance(x, (stl.ast.G, stl.ast.And)) for x in phi2.walk())
diff --git a/stl/utils.py b/stl/utils.py
index f49b960..6e46efd 100644
--- a/stl/utils.py
+++ b/stl/utils.py
@@ -5,7 +5,9 @@ import traces
 from lenses import bind
 
 import stl.ast
-from stl.ast import (And, F, G, Interval, LinEq, Neg, Or, AP_lens)
+from stl.ast import (And, F, G, Interval, LinEq, Neg,
+                     Or, AP_lens, Next, Until, AtomicPred,
+                     _Top, _Bot)
 from stl.types import STL
 
 
@@ -13,24 +15,28 @@ oo = float('inf')
 
 
 def f_neg_or_canonical_form(phi: STL) -> STL:
-    if isinstance(phi, LinEq):
+    if isinstance(phi, (LinEq, AtomicPred, _Top, _Bot)):
         return phi
 
     children = [f_neg_or_canonical_form(s) for s in phi.children]
     if isinstance(phi, (And, G)):
         children = [Neg(s) for s in children]
-    children = tuple(children)
+    children = tuple(sorted(children, key=str))
 
     if isinstance(phi, Or):
         return Or(children)
     elif isinstance(phi, And):
         return Neg(Or(children))
     elif isinstance(phi, Neg):
-        return Neg(children[0])
+        return Neg(*children)
+    elif isinstance(phi, Next):
+        return Next(*children)
+    elif isinstance(phi, Until):
+        return Until(*children)
     elif isinstance(phi, F):
-        return F(phi.interval, children[0])
+        return F(phi.interval, *children)
     elif isinstance(phi, G):
-        return Neg(F(phi.interval, children[0]))
+        return Neg(F(phi.interval, *children))
     else:
         raise NotImplementedError